From Latin 'pēnis', which originally meant 'tail' before its anatomical sense became primary. Related to 'pencil' (Latin penicillus = little tail/brush) and 'penicillin' (named for the brush-shaped mold). All three are diminutives of the same Latin tail.
The male organ of copulation and urination in humans and other mammals.
Borrowed directly from Latin pēnis, which originally meant 'tail' before acquiring its anatomical sense through colloquial euphemism. The Latin word likely derives from PIE *pes- (penis, tail), though some scholars connect it to *pen- (to hang, to droop, to swing), which would make it cognate with English pendant and Latin pendēre (to hang). Latin pēnis as 'tail' is attested in Cicero, Horace, and other classical authors alongside its anatomical usage; the two senses
Latin 'pēnis' (tail) is hiding inside 'pencil' — from Latin 'penicillus' (little tail, fine brush), a diminutive of 'peniculus' (brush, sponge), itself from 'pēnis' (tail). And 'penicillin' was named after the brush-like shape of the Penicillium mold. So pencil, penicillin, and penis are all etymological relatives — all little tails.