1Requiring, involving, or characterized by hard work or exertion.
2 rare Of a ship: unstable; inclined to pitch and roll violently in rough seas; (of a cargo) liable to cause a ship to become unstable in this way. Compare "labour".
Mid 16th century; earliest use found in Ralph Robinson (1520–1577), translator. Partly from labour + -some, and partly from labour + -some.