biceps

1630s (adj.) "two-headed," specifically in anatomy, "having two distinct origins," from Latin biceps "having two parts," literally "two-headed," from bis "double" (see bis-) + -ceps, combining form of caput "head" (from PIE root *kaput- "head"). As a noun meaning "biceps muscle of the arm," from 1640s, so called for its structure. Despite the -s, it is singular, and classicists insist there is no such word as bicep.

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