of

of O.E. of, unstressed form of æf (prep., adv.) "away, away from," from P.Gmc. *af (Cf. O.N. af, O.Fris. af, of "of," Du. af "off, down," Ger. ab "off, from, down"), from PIE *apo- "off, away" (see APO- (Cf. apo-)). Primary sense in Old English still was "away," but shifted in Middle English with use of the word to translate L. de, ex, and especially O.Fr. de, which had come to be the substitute for the genitive case. "Of shares with another word of the same length, as, the evil glory of being accessory to more crimes against grammar than any other." [Fowler]

Also from 1837 a non-standard or dialectal representation of have as pronounced in unstressed positions (could of, must of, etc.)