Daily entries from the 17th century London diary
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pinner, a coif with 2 long flaps, one on each side, pinned on and hanging down, and sometimes fastened at the breast; worn by women, esp. of rank, in the 17th and 18th centuries. Sometimes applied to the flaps as an adjunct of the coif.
definition from the OED.
To add to AFagins’ comments, a Pinner can also refer to a fill-in above a Decolletage ( a very low cut dress that reveals cleavage)