means A male chicken, especially an adult one kept for breeding or, famously, for its loud dawn crowing.

from A plainly American coinage: an agent noun from 'roost,' literally 'one that roosts,' i.e., 'the bird that perches.' It rose to favor as a polite substitute for the older word 'cock,' which had grown awkward in genteel mouths. 'Roost' itself goes back to Old English 'hrost,' a perch or framework where birds settle for the night — so a rooster is, etymologically, simply a sleeper-on-a-pole, which makes its reputation for raucous early rising slightly ironic.