"of which, of whom," c. 1200, from where (in the sense of "in which position or circumstances") + of. Similar formation in Swedish hveraf, Danish hvoraf, Dutch waarvan.
Entries linking to whereof
where adv.
Old English hwær, hwar "at what place," from Proto-Germanic adverb *hwar (source also of Old Saxon hwar, Old Norse hvar, Old Frisian hwer, Middle Dutch waer, Old High German hwar, German wo, Gothic hvar "where"), equivalent to Latin cur, from PIE root *kwo-, stem of relative and interrogative pronouns. Where it's at attested from 1903.
It has figured in a great many prepositional and adverbial compounds through the years; in addition to the ones listed in this dictionary (whereas, wherefore, whereabouts, etc.) English has or had whereagainst, wherefrom, wherehence, whereinsoever, whereinto, wheremid, whereout, whereover, whereso, wheresoever, wherethrough, whereto, whereunder, whereuntil.
of prep.
Old English of, unstressed form of æf (prep., adv.) "away, away from," from Proto-Germanic *af (source also of Old Norse af, Old Frisian af, of "of," Dutch af "off, down," German ab "off, from, down"), from PIE root *apo- "off, away."
The primary sense in Old English still was "away," but it shifted in Middle English with use of the word to translate Latin de, ex, and especially Old French 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 by 1837 of in print could be a non-standard or dialectal representation of have as pronounced in unstressed positions (could of, must of, etc.)