To grouse, meaning to complain, is 19th-century British army slang, probably derived by some circuitous route from the Greek word gruzein, to grumble. A grouse, meaning a wild, jaunty-looking, and unfortunately (for it) delicious game bird, has no relationship at all to grumbling, at least not that I've been able to discover.
I sure wasn't grumbling when I spent a few days in London recently and managed to eat grouse three different times in as many days. The first was at Hix in Soho, one of chef Mark Hix's five no-nonsense local establishments, where I had a pretty much perfect meal of West Mersea and Duchy of Cornwall native oysters followed by a roasted grouse served with the traditional accompaniments of rowan (sorb apple) jelly, "game chips" (potato chips to you) and that peculiar but somehow reassuring ancient British condiment called bread sauce (which is basically cream sauce thickened with breadcrumbs).
The second and third times came two nights later, when a group of us devoured almost everything on the menu at St. John, the original "nose to tail" restaurant near the wholesale Smithfield meat market—including (among many other things) salads of snails, grilled ox heart, and lamb's tongue with green beans and anchovies, followed by roasted mallard, roasted grouse, and a savory pie of grouse and pigs' trotters.
The grouse we're talking about here is the red grouse (Lagopus lagopus scotica), which exists today only in the British Isles and Ireland, and even there mostly on the moors on Scotland and Northern England. This accounts for its alternate name, moorbird (or moorhen). Grouse-hunters await "the Glorious Twelfth"—the twelfth of August, which is the official start of grouse season in the British Isles—with all the giddy eagerness of five-year-olds counting down the days until Santa squeezes down the chimney. (Why the twelfth? Because that's the date on which Queen Victoria took up residence at Balmoral Castle in Scotland for her annual sporting holiday.)
It is not true that, as some grouse aficianados will tell you, we don't have grouse in America; we just don't have red grouse. Ours is ruffed grouse (Bonasa umbellus), sometimes mistakenly identified as partridge, and devourers of the British variety will tell you that it's a pretty sorry substitute. So sorry, in fact, that in Kentucky and Ohio, it is sometimes known as grinch.
I've never eaten ruffed grouse, but I can report that the meat of red grouse is rich and earthy, with a faint herbaceous flavor that probably reflects its diet, which is almost entirely heather; it does not, in other words, taste like chicken. Grouse is undeniably "gamy"—because, well, it's game. This effect is heightened by hanging—i.e., suspending the birds in a cool place until they begin to putrefy. Hanging tenderizes the flesh and lends it a pungent, bitterish character much appreciated in some quarters and rather enthusiastically eschewed in others. I like the effect in moderation (the roasted grouse at St. John was too "high" for my taste). Moderately aged and correctly roasted (i.e. juicy and very pink though not bloody), it really is one of the great old-fashioned pleasures of the autumn table.