pinafore n.

mid-14c., pinnen, "to affix with a pin," from pin (n.). Figurative uses, on the notion of "seize and hold fast in the same spot or position" are from 1570s. Related: Pinned; pinning. Sense of "to hold someone or something down so he or it cannot escape" is attested from 1740. In U.S. colleges, as a reference to the bestowal of a fraternity pin on a female student as an indication of a relationship, it is attested by 1938. Phrase pin down "define" is from 1951.

Middle English, from Old English onforan, contraction of prepositional phrase on foran "before in place, at the beginning of, in front of," from on (prep.), see a- (1), + foran (adv.) "in front," dative of for. In some cases probably it represents Old English ætforan "at-fore."

Attested from early 14c. as a preposition, "before in time," and as a conjunction, "earlier than the time when, before." Once the literary equivalent of before, it now has been replaced by that word except in nautical use, colloquial dialects, and in combinations such as aforesaid, aforethought.