Is it ensure, insure, or assure?
1580s, of imitative origin (cf. Dutch and East Frisian klikken "to click; Old French clique "tick of a clock"). The figurative sense, in reference usually to persons, "hit it off at once, become friendly upon meeting" is from 1915, perhaps based on the sound of a key in a lock. Related: Clicked; clicking.
1610s, from click (v.). Click-beetle attested from 1830.
A slight sharp sound, such as that heard from the heart during systole.