Daily entries from the 17th century London diary
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A Jacobus is an English gold coin of the reign of James I, worth 25 shillings.
The correspondence of Isaac Newton refers to the coin:
The Jacobus piece coin'd for 20 shillings is the 41th: part of a pound Troy, and a Carolus 20s piece is of the same weight. But a broad Jacobus (as I find by weighing some of them) is the 38th part of a pound Troy.
More broadly, Jacobus is derived from the bible name Jacob which means the wise one
A jacobus was a gold coin of the value of twenty-five shillings, called after James I, in whose reign it was first coined.
from L&M Large Glossary
JACOB(US)
gold sovereign coined under James I.
OED:
The current (but not official) name of an English gold coin, struck in the reign of James I.
Originally issued in 1603, under the name of the Sovereign, and current for 20s. In 1604 there was a second issue known as the Unite, which being lighter, the value of the Sovereign rose to 22s. In 1612 the current value of the Unite was raised by statute to 22s., and the earlier piece rose to 24s.
1612 in Crt. & Times Jas. I (1849) I. 197 The prince having entreated him to provide him £1000, in so many Jacobus pieces. a1618 RALEIGH Obs. in Rem. (1661) 200 The English Iacobus goeth for three and twenty shillings in Merchandizing. […]
(The stress is on the second syllable: ja-CO-bus.)