"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]
late 14c., Latin, "you may know, you may be sure, it is certain," used in sense "that is to say, namely," contraction of scire licit "it is permitted to know," from scire "to know" (see science); for second element see licit. Used as was Old English hit is to witanne, literally "it is to wit" (see wit (v.)). Often abbreviated sc. or scil.
Its function is to introduce : (a) a more intelligible or definite substitute, sometimes the English, for an expression already used ... (b) a word &c. that was omitted in the original as unnecessary, but is thought to require specifying for the present audience .... [Fowler]
"Inward awareness of right or wrong" (a word formed to translate Latin conscientia), early 13c., "conscience;" c. 1300, "reason, intellect," from in (adj.) + wit (n.). Not related to Old English inwit, which meant "deceit." Joyce's use of it in "Ulysses" (1922) echoes the title of the 14c. work "Ayenbite of Inwyt" ("Remorse of Conscience," a translation from French) and is perhaps the best-known example of the modern use of the word as a conscious archaism, but it is not the earliest.
Þese ben also þy fyve inwyttys: Wyl, Resoun, Mynd, Ymaginacioun, and Thoght [Wyclif, c. 1380]
If ... such good old English words as inwit and wanhope should be rehabilitated (and they have been pushing up their heads for thirty years), we should gain a great deal. [Robert Bridges, English poet laureate, 1922]