"to know, be certain about, have knowledge of" (archaic), Old English witan (past tense wast, past participle witen) "to know, beware of or conscious of, understand, observe, ascertain, learn," from Proto-Germanic *witanan "to have seen," hence "to know" (from PIE root *weid- "to see;" compare wise (adj.)).

The phrase to wit, almost the only surviving use of the verb, is recorded by 1570s, from earlier that is to wit (mid-14c.), itself probably a loan-translation of Anglo-French cestasavoir, used to render Latin videlicet (see viz.).

Germanic cognates include Old Saxon witan, Old Norse vita, Old Frisian wita, Middle Dutch, Dutch weten, Old High German wizzan, German wissen, Gothic witan "to know."

God wot "God knows," used to emphasize truth, is by early 13c. Also see wist.

late 15c., "admit or show one's knowledge," a blend of Middle English aknow "admit or show one's knowledge" and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). Middle English aknow is from Old English oncnawan "understand, come to recognize," from on (see on (prep.)) + cnawan "recognize" (see know).

"By 16th c. the earlier vbs. knowledge and a(c)know ... were obs., and acknowledge took their place" [OED, 1989]. In the merger, an unetymological -c- slipped in; perhaps the explanation is that when English kn- became a simple "n" sound, the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.

c. 1300, "to become acquainted with; to be aware or conscious of" (obsolete), from Old English becnawan "to know," or a Middle English formation from be- + know (v.). Related: Beknown; beknowing.

Old English cneow, past tense of know (v.).

c. 1400, from know (v.) + -able.

also knowhow, "technical expertise," 1838, American English, from know (v.) + how (adv.).

"with knowledge of truth," late 14c., present-participle adjective from know (v.). From c. 1500 as "shrewd, sharp, smart." Related: Knowingly.

"one deemed (over)full of information or correct answers," 1895, from verbal phrase; see know (v.). Earlier in the same sense was know-all (1862); and Mr. Know-All was a minor character in Bunyan's "The Holy War" (1682).

early 12c., cnawlece "acknowledgment of a superior, honor, worship;" for the first element see know (v.). The second element is obscure, perhaps from Scandinavian and cognate with the -lock "action, process," found in wedlock.

From late 14c. as "capacity for knowing, understanding; familiarity;" also "fact or condition of knowing, awareness of a fact;" also "news, notice, information; learning; organized body of facts or teachings." The sense of "sexual intercourse" is from c. 1400. Middle English also had a verb form, knoulechen "acknowledge" (c. 1200), later "find out about; recognize," and "to have sexual intercourse with" (c. 1300); compare acknowledge.

Since Knowledge is but sorrow's Spy,

It is not safe to know.

[Davenant, song from "The Just Italian," 1630]

mid-13c., knouen, "well-known, famous, notorious," past-participle adjective from know (v.). From early 14c. as "recognized, not secret; familiar, not strange." As a noun, "that which is known," by 1863; earlier "famous person" (1835). In Middle English it meant "one's acquaintances." To make (something) known is from mid-14c.