I was under the impression it was a nonsense word, and then I ran into it in D. H. Lawrence's The Rainbow. In the first chapter, when Tom Brangwen is still just a teenager, his older brother Frank, a butcher, calls him a "mardy baby." I have a moderately vintage Compact Oxford English Dictionary, where I have to look up every word twice, because the supplements are printed separately from the old first edition. In the first-edition part, I found mard listed as a variant of marred, merd, or marter. Merd derives from the French merde, and means the same thing, so the phrase "mardy bum" suddenly took on a vividly blue cast. But this turned out to be a wrong turn, fortunately. In the supplement, mardy appeared in its own right, defined as " Spoilt', sulky, whining," and exemplified by no fewer than three quotations from . . . D. H. Lawrence, who seems to have absolutely adored the word, because quotes from him also illustrate mard as a verb ("She marded 'em till they were soft") and as an adjective ("Eh, tha'rt a mard-'arsed kid"). The OED claims the word comes from "marred"---i.e., spoiled---and that the French have nothing to do with it.
UPDATE (3/25): Mitch Gould writes to suggest that perhaps mardy does come from merde after all, and that the OED editors have cleaned up the etymology, as it were. This would make the mard-from-marred derivation the opposite of a folk etymology, I suppose. (Is there a word for that?) As Mitch points out, the fact that mard appears with both bum and arse is awfully suspicious, given that there's nothing especially sulky about that part of the body. Also, if mard came from marred, it's strange that anyone would feel obliged to add a -y suffix to it in order to make it sound adjectival; marred is perfectly capable of functioning as an adjective solo.
