con (n.1, adv.)

Old English 1st and 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "have power to, to be able," (also "have carnal knowledge"), from Proto-Germanic *kunnjanan "be mentally able, have learned" (source also of Old Norse kenna "become acquainted, try," Old Frisian kanna "recognize, admit, know," German kennen "know," Middle Dutch kennen "know," Gothic kannjan "make known"), from PIE root *gno- "to know."

It holds now only the third sense of "know," that of "know how to do something" (as opposed to "know as a fact" and "be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.

c. 1400, "assurance or belief in the good will, veracity, etc. of another," from Old French confidence or directly from Latin confidentia, from confidentem (nominative confidens) "firmly trusting, bold," present participle of confidere "to have full trust or reliance," from assimilated form of com, here perhaps an intensive prefix (see com-), + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade").

From mid-15c. as "reliance on one's own powers, resources, or circumstances, self-assurance." Meaning "certainty of a proposition or assertion, sureness with regard to a fact" is from 1550s. Meaning "a secret, a private communication" is from 1590s. The connection with swindling (see con (adj.)) dates to mid-19c. and comes from the notion of the false "trustworthiness" which is the key to the game.