the.com/cock

means A male chicken, or by extension any stopcock, tap handle, or trigger-mechanism part shaped like one — plus the slang sense everyone's actually thinking of.

from From Old English 'cocc,' almost certainly echoic — imitating the bird's own crowing cry, like many words for roosters across unrelated languages. The faucet/tap sense grew naturally from the rooster's strutting, spigot-like profile and its role as the lever you turn to let something flow (hence 'stopcock' and 'cock the gun,' from pulling back a part that juts out like a rooster's head). The slang sense arrived later, riding on the same jutting, cocky silhouette — a bit of linguistic innuendo so old it's practically barnyard folklore in its own right.