Answer: Gentleman cow.
My grandmother gets American Heritage and passes them on to me when I see her, which is, unfortunately, infrequently. So stop me if you've heard this one.
A regular column in AH is "Why Do We Say...?" where a number of idiosyncratic American phrases are pointed out and etymologized for your readerly edification. In the August/September '06 issue, they took apart rooster. Apropos, no?
Apparently "rooster," along with a host of words, was used as a polite replacement for the unseemly--nay, scandalous!--(though not incorrect) "cock." Appearing during the end of the 1700's, people were getting more particular about vulagrities in speech as such, as AH points out:
This is the period when bosom, limb, and donkey replaced breast, leg, and ass; when breeches and trousers became inexpressibles, unmentionables, and nether garments; when died was superseded on gravestones by passed away, laid to rest, and fell asleep; and when the sexually potent barnyard bull was converted into the cow brute, cow’s spouse, and gentleman cow.
The comedy of "gentleman cow" is obviously a result of presentism. In this era where you can see (and hear) pretty much whatever you want on television, it is amusing to see that some terms whose origins remain obscured from popular culture display a long-felt tradition of propriety. In fact oftentimes there is that certain amusement: I've heard my mother, for example, say "unmentionables" with a knowing laugh, as if she is tickled by the not-too-clever subterfuge of that now-antiquated term.
The male chicken, though, is an interesting exception. Most schoolyard kids know that there is a naughty way to say it, but no one giggles at "rooster." The term has become an imbued with as much currency as the original double entendre. Part of the reason may have as much to do with the ubiquity of the syllable "cock" in even the most innocuous places that added to a heightened sensitivity. You just can't escape it, so try as you might, you need a valid replacement. The rooster word at least makes some sense as the cock was most often seen "roosting"--a term already in use--on the family barn. The case could be made that the descriptive element added to that currency. Nonetheless, the term "cock" was contentious. Even Louisa May Alcott's father Amos, AH says, had changed the family's name from Alcox, an even less obvious reference than the earlier Alcocke, or the even earlier (though only conjecture; records don't go back far enough) just Alcock.
Talk about a different society. Hardly anybody bats an eye at naming your child Peter or Richard (yes, Dick) or even something more unfortunate. And even if they do, it's surely not enough to go and change your name. Nonetheless, AH points out that language is pretty tough to unattach from extant association: "[W]e continue to say rooster (as well as donkey and haystack) and, when presented with the rooster’s spouse at the dinner table, we are likely to ask for white meat or dark meat instead of breast or thigh and for a drumstick instead of a leg. Thus we honor our ancestors’ hang-ups." I guess my family was low-class: when I was a kid, I only ate the legs because I didn't know the difference--nobody used those terms.
Of course the whole thing is cute, which makes good copy. It also, however, points to the way we think about the things that surround us, and how some everyday items get bound up in semantic chicanery that will make your grandmother blush. Unfortunately, that's the part that gets left out of the article (no fault of theirs as it's only a page-long feature) that would be really interesting to know: where did these terms originate that made our forebears silly with embarrassment? Was "cock" already a euphemism when the animal got attached to it or vice-versa? What could a bird do to get saddled with that? The cock is obviously the male rooster and (euphemistically) the male sex organ, and both reinforce some invocation of masculinity, gender, and power (note the logo for Rooster Records, above left). I've even heard the term "top cock" mean the same as "the guy in charge" in fairly polite conversation--and no, not as a slip-up. But the question remains: which came first, the chicken or the--
Nevermind. I think you get the point.
As a final note, check out issue 32:2 (1957) of the journal American Speech for H.D. Rowe's near-hilarious romp, "New England Terms for 'Bull': Some Aspects of Barnyard Bowlderism," available on JSTOR. He cites some 42 different euphemisms for "bull" in the 1930's. You don't get phrases like this much in academic writing: "One shudders to think of the repressed dirtiness of mind implicit in these substitutions" (actually, a quote from a 1933 work Rowe employs). Maybe he forgot about Calvinism. A drier, though no less interesting, take is Mencken's The American Language (1921), now out of copyright and available numerous places on-line.