"to know" (archaic), c. 1500, from Old English past tense of witan "to know" (cognate with German wusste, past tense of wissen "to know"); see wit. Had-I-wiste was used c. 1400-1550 in sense "regret for something done rashly or heedlessly;" see wist. Proverbial in expression Had-I-wiste cometh ever too late.
Haddywyst comyth euer to late Whan lewyd woordis beth owte y-spronge. ["Commonplace book" in Trinity College, Cambridge, c. 1500]