"worthless sponging idler," 1863, American English slang, perhaps originally Civil War slang, from dead (adj.) + beat. Earlier used colloquially as an adjectival expression to mean "completely beaten" (1821), and perhaps the base notion is of "worn out, good for nothing." It is noted in a British source from 1861 as a term for "a pensioner."
It also was used of a kind of regulating mechanism in pendulum clocks.