c.1300, "a task, a project;" later "exertion of the body; trouble, difficulty, hardship" (late 14c.), from Old French labor "labor, toil, work, exertion, task" (12c., Modern French labeur), from Latin laborem (nominative labor) "labor, toil, exertion; hardship, pain, fatigue; a work, a product of labor," of uncertain origin, perhaps originally from the notion of "tottering under a burden," and related to labere "to totter."
Meaning "body of laborers considered as a class" (usually contrasted to capitalists) is from 1839. Sense of "physical exertions of childbirth" is 1590s, earlier labour of birthe (early 15c.), a sense also found in Old French, and cf. French en travail "in (childbirth) suffering" (see travail). Labor Day first marked 1882 in New York City.