deadbeat (n.) "worthless sponging idler," 1863, American English slang, perhaps originally Civil War slang, from dead (adj.) + beat. Earlier dead beat was used colloquially as an adjectival expression, "completely beaten, so exhausted as to be incapable of further exertion" (1821), and perhaps the base notion is of "worn out, good for nothing."

Probably from the slang verb 'to beat' (to cheat)..."Deadbeat, another word for loafer or sponger, also came into vogue during the Civil War. The first syllable probably comes from the adjective 'dead' ('complete'), as in 'dead broke.'" Another reference has "deadbeat" as a verb in the 1880s.