late 14c., "perform manual or physical work; work hard; keep busy; take pains, strive, endeavor" (also "copulate"), from Old French laborer "to work, toil; struggle, have difficulty; be busy; plow land," from Latin laborare "to work, endeavor, take pains, exert oneself; produce by toil; suffer, be afflicted; be in distress or difficulty," from labor "toil, work, exertion" (see labor (n.)).
The verb in modern French, Spanish, and Portuguese means "to plow;" the wider sense being taken by the equivalent of English travail. Sense of "endure pain, suffer" is early 15c., especially in phrase labor of child (mid-15c.). Meaning "be burdened" (with trouble, affliction, etc., usually with under) is from late 15c. The transitive senses have tended to go with belabor. Related: Labored; laboring.