con (n.1, adv.)

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.

word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.

Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.