When one's breath smells like cock, one is said to have "Cock Breath." One who is said to have "Cock Breath" took on the smell of cock because s/he performs fellatio.
"Cock breath" is usually used as an insult, with the inference is that one either performs fellatio often or has performed it recently, because it is "on" one's breath. For men, this can be degrading as it implies he is a homosexual or otherwise likes to suck cock. For women, this can be degrading in that it implies she sucks alot of cock and is therefore a slut.
"Cock Breath" can also be used interchangibly with "cocksucker" and "dicksucker." Many would say "Cock breath" is more original and is therefore a funnier insult.
as a characterization of one's breath: "That dude's got massive case of cock breath, probably from sucking some massive cock."
as a name: "Hey, Cock Breath, I got something in my shorts for you!"