Bow-wow mutton is mutton that barks. Put less politely: dog meat, or meat so bad you suspect it.

It appears on lists of Victorian slang, and J. Redding Ware did record it in 1909 — as sailors' slang for meat so bad that it might be dog-flesh. But the phrase was already old by then.

Francis Grose's Classical Dictionary of the Vulgar Tongue had it in 1785, and the 1811 edition still carries the entry in three words: BOW-WOW MUTTON. Dog's flesh. Grose was not describing bad cooking. He meant it literally — or meant that people said it literally, which in a slang dictionary amounts to the same thing.

The Victorians kept the joke going, and its usual target was the London pieman. Green's Dictionary of Slang quotes an 1834 report in the Morning Post in which a pie-seller complains that his pies are made of very best Southdown mutton, and that it is very hard to be picked out and called a bow-wow dealer just because some other sellers were. In 1858 Augustus Mayhew's Paved with Gold offers the reassurance that, if you're hungry, you can be certain of not chewing 'bow-wow mutton'.

So the phrase belongs to a long English tradition of distrusting anything minced, wrapped or baked by a stranger — the same tradition that called sausages bags o' mystery. Whether the dog was ever really in the pie is another question. The suspicion, at least, is well documented.