This seems to be an old term for marmalade. There are some recipes for marmalet from 1658 here.
From "Pepys At Table" by Driver and Berriedale-Johnson (p 110-111)
"Preserve making has always been catching and Elizabeth was obviously eager to try her hand with quinces after her session with Mrs. Hunt. A seventeenth-century 'marmalett' or marmalade could be made from any fruit which was boiled and crushed with sugar to help preserve it; the exclusive link between marmalade and oranges is of comparatively recent origin.
COTIGNACS AND MARMALADE OF QUINCES
Giles Rose "A Perfect School of Instruction for the Officers of the Mouth" 1682
Take what fruit you please, cut it in quarters and boyl it; and when it is boyled let the water run well from it, then strain it through a colander or Hair Strainer, then boyl half a pound of sugar a Soufle and being boiled, put into a pound of your Marmalad, first taking it from the fire, and let it stand till it be cool, and when it is cold dress or fashion it upon a plate and mark it as you do other pastes. This may serve to make a Tart either laced or covered.
Recipe [today's translation]
1 lb / 450 g quinces, pears, apples or other hard fruit
8 ozs / 225 g sugar
Peel the fruit, quarter or slice it roughly, put it in a saucepan just covered with water, bring to a boil and simmer till it is well cooked. Drain the fruit, then purée it either in a food processor or through a sieve. Drain the fruit very well once it is pureed. In a wide pan melt the sugar and allow it to cook until it is just beginning to turn colour. Remove it from the heat immediately and add the fruit purée to it. Stir the mixture well and allow to cool. It may then be 'shaped' and eaten alone or used as a filling for a fruit tart or pie."
cum salis grano • Link
OED: [< Portuguese marmelada quince marmalade (first attested 1521) < marmelo quince (first attested 1527, but cf. e.g. marmeleira quince-orchard (973)) + -ada -ADE. Portuguese marmelo is < post-classical Latin malomellum quince or sweet apple (Isidore, who suggests that the name may refer to the sweetness of the fruit or its being served with honey), blend (with change of vowel in first syllable after classical Latin m-a-la see male of classical Latin m-e-lum: see MALE n.2) of classical Latin m-e- lomeli honey flavoured with quinces (< Hellenistic Greek [* melomeli] (Dioscorides)) and classical Latin melim-e-la (plural) a variety of sweet apple (< Hellenistic Greek .. [* melomeli]......summer apple ....... , in Byzantine Greek also apple grafted on quince). Both Hellenistic Greek words are < ancient Greek [gr] apple or similar fruit (see MALE n.2; .[ greek word] ..means specifically 'quince': see MELOCOTON n., COYN n.) [G-meli*] +honey (see MELL n.2).
[* Pinch of salted greek]
Close medieval trading relations between England and Portugal may account for the very early borrowing of the Portuguese word in English: cf. Middle French marmeline (1541), Spanish mermelada (1570), Italian marmellata (1579), Middle French mermelade (1573), French marmelade (1602, marmelat (1605), Swedish marmelad (1578), post-classical Latin marmelatum (1588, in a French text), German Marmelade (c1600, perh. < French), Dutch marmelade (1599).]
A. n.I. Simple uses.
1. a. Originally:
a preserve consisting of a sweet, solid, quince jelly resembling chare de quince (see CHARE n.4) but with the spices replaced by flavourings of rosewater and musk or ambergris, and cut into squares for eating; (in the 17th cent., occas.) a thick, apple-based jelly containing shredded citrus peel (obs.). Subsequently: a conserve made by boiling fruits (now usually oranges and other citrus fruits) in water to release the pectin around the seeds, then reboiling the liquid and fruit with sugar to form a consistent mass, typically containing embedded shreds of rind. Also: a preparation of similar consistency made with other ingredients, such as a sweet preserve of diced ginger in a jelly set with apple pectin, or a relish made by cooking vegetables with sugar and vinegar.
Often with the name of the fruit or other dominant ingredient prefixed, as apricot, ginger, lemon, onion, orange, quince marmalade. When none is specified, orange marmalade is now usually meant; this may then be prefixed by a word denoting the style or type of orange marmalade, as diabetic, Dundee marmalade. Oxford marmalade: see OXFORD n. 1a.
Since 1981, European Community regulations have restricted commercial use of the term to preserves made with citrus fruit.
1621 R. BURTON Anat. Melancholy II. II. I. i. 308 Marmalit of plummes, quinces &c.
1634 T. HERBERT Relation Trav. 168 A healing powder of Salt and Marmalate of Dates.
a1676 DUKE OF NEWCASTLE Humorous Lovers (1677) IV. ii. 42 What an admirable thing it is for a Lady..to be skilfull in the great secret of Preserving, making Marmalads, Quidenies and Gellies.
b. fig. and in extended use. Earliest in marmalade and sucket (see SUCKET n.). 1592
2. The fruit of the mammee sapota of Central America, Pouteria sapota, which is said to taste like quince marmalade; (also) the tree itself. More fully American marmalade, natural marmalade. Cf. marmalade plum, sense 3a. Obs.
[1604 E. GRIMESTON tr. J. de Acosta Nat. & Morall Hist. Indies IV. xxv. 257 There is a kinde of fruite like vnto Cotignac, or marmelade.]
aside ...marmalade-madam Obs., a prostitute.
1674 J. JOSSELYN Acct. Two Voy. 162 The Gallants a little before Sun-set walk with their *Marmalet-Madams, as we do in Morefields
. Sweet. Obs.
1 sweet Obs
1617 H. ROBERTS Pheander Mayden Knight xxiii. sig. Ov, Nothing worthy to enioy thee so marmalade a lasse.
1630 P. MASSINGER Picture I. i. 101, I cannot blame my ladies Vnwillingnesse to part with such marmulade lips.
not be confused with Marmite [< Middle French, French marmite cooking pot (14th cent.), of uncertain origin (cf. post-classical Latin marmita (1318-19 in a British source) < French): the theory that it is < Old French marmite hypocrite, on the grounds that it conceals its contents, is not convincing. Sense 2 is recorded in French from 1847 (earlier 1637 'bombes ou grosses grenades..en forme de marmites de fer'), and became common in First World War (1914-18).
The m t/; the m mi t/ was ever current.]
1. orig. Brit. regional and Mil. A cooking pot of metal or (now usually) of earthenware; a stockpot.
1581 W. FOWLER Wks. (1936) II. 47 It is a grit pitie..thou want a pulpet, hauing sa weil preachit ouer the pot. I think this reason was maid in the marmet
[< marm- (in ancient Greek [* marmai reo] to shine
Ruben • Link
"According to the Oxford English Dictionary, "marmalade" appeared in English in 1480, borrowed from French marmelade which, in turn, came from the Portuguese marmelada...There is no truth whatsoever to the folk etymology which states that the word derives from "Marie malade" (French for "ill Mary"), referring to Mary, Queen of Scots, because she used it as a medicine for a headache or upset stomach."
"The Romans learned from the Greeks that quinces slowly cooked with honey would "set" when cool (though they did not know about fruit pectin). Greek melimelon or "honey fruit"--for most quinces are too astringent to be used without honey, and in Greek "melos" or "apple" stands for all globular fruits--was transformed into "marmelo." The Roman cookbook attributed to Apicius gives a recipe for preserving whole quinces with their stems and leaves attached in a bath of honey diluted with defrutum: Roman marmalade."
"The extension of "marmalade" in the English language to refer to citrus fruits was made in the 17th century, when citrus first began to be plentiful enough in England for the usage to become common."
Chart showing the number of references in each month of the diary’s entries.