late 12c., "indolence, sluggishness," formed from Middle English slou, slowe (see slow (adj.)) + abstract formative -th (2). Replaced Old English slæwð "sloth, indolence." Sense of "slowness, tardiness" is from mid-14c. As one of the deadly sins, it translates Latin accidia.
The slow-moving mammal first so called 1610s, a translation of Portuguese preguiça "slowness, slothfulness," from Latin pigritia "laziness" (cf. Spanish perezosa "slothful," also "the sloth").