Origin and history of well-known

Old English cnawan (class VII strong verb; past tense cneow, past participle cnawen), "perceive a thing to be identical with another," also "be able to distinguish" generally (tocnawan); "perceive or understand as a fact or truth" (opposed to believe); "know how (to do something)," from Proto-Germanic *knew- (source also of Old High German bi-chnaan, ir-chnaan "to know"), from PIE root *gno- "to know."

For pronunciation, see kn-. Once widespread in Germanic, the verb is now retained there only in English, where it has widespread application, covering meanings that require two or more verbs in other languages (such as German wissen, kennen, erkennen and in part können; French connaître "perceive, understand, recognize," savoir "have a knowledge of, know how;" Latin scire "to understand, perceive," cognoscere "get to know, recognize;" Old Church Slavonic znaja, vemi). The Anglo-Saxons also used two distinct words for this, the other being witan (see wit (v.)).

From c. 1200 as "to experience, live through." The meaning "to have sexual intercourse with," also found in other modern languages, is attested from c. 1200, from the Old Testament (Genesis iv.1). Attested from 1540s in colloquial phrases suggesting cunning or savvy (but often in the negative).

As far as (one) knows "to the best of (one's) knowledge" is late 14c. Expression God knows is from c. 1400. To know too much (to be allowed to live, escape, etc.) is from 1872. To know better "to have learned from experience" is from 1704.

As an expression of surprise, what do you know attested by 1914. Don't I know it in the opposite sense ("you need not tell me") is by 1841.

Middle English wel, "in a satisfactory or pleasing manner; laudably, properly," used very broadly to indicate successful accomplishment of the action in the verb it accompanies; from Old English wel "abundantly, very, very much; indeed, to be sure; with good reason; nearly, for the most part."

This is from Proto-Germanic *wel- (source also of Old Saxon wela, Old Norse vel, Old Frisian wel, Dutch wel, Old High German wela, German wohl, Gothic waila "well"), which in Watkins is reconstructed to be from PIE root *wel- (2) "to wish, will" (source also of Sanskrit prati varam "at will," Old Church Slavonic vole "well," Welsh gwell "better," Latin velle "to wish, will," Old English willan "to wish;" see will (v.)).

It has come to be used as the adverb of good. From late 12c. as "in a state of good health, soundly." Also used in Old English as an interjection and an expression of surprise. By mid-12c. as a mere intensifier (may well be).

Expression might as well (do something, as not do it, or as do something else) is by late 14c., as is as well as "in addition to."