late 14c., "a thought, a notion, that which is mentally conceived," from conceiven (see conceive) based on analogy of deceit/deceive and receipt/receive. Sense evolved from "something formed in the mind" to "fanciful or witty notion, ingenious thought" (1510s), to "vanity, exaggerated estimate of one's own mental abilities" (c. 1600) through shortening of self-conceit (1580s).
A doublet of concept, it sometimes was spelled conceipt in Middle English. Sometimes the Italian form concetto (plural concetti) was used in English 18c.-19c. for "piece of affected wit;" OED describes it as "a term originally proper to Italian literature."