1630s (adj.), from Latin biceps "having two parts," literally "two-headed," from bis "double" (see bis-) + -ceps comb. form of caput "head" (see capitulum). As a noun meaning "biceps muscle," from 1640s, so called for its structure. Despite the -s, it is singular, and classicists insist there is no such word as bicep.
biceps bi·ceps (bī'sěps')
n. pl. biceps or bi·ceps·es (-sěp'sĭz)
A muscle with two heads or points of origin.
The large muscle at the front of the upper arm that flexes the forearm.
The large muscle at the back of the thigh that flexes the knee joint.