E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who carps at everything. Momus, the sleepy god, was always railing and carping.
Momus, being asked to pass judgment on the relative merits of Neptune, Vulcan, and Minerva, railed at them all. He said the horns of a bull ought to have been placed in the shoulders, where they would have been of much greater force; as for man, he said Jupiter ought to have made him with a window in his breast, whereby his real thoughts might be revealed. Hence Dr. Gray says that every unreasonable carper is called a Momus.