mid-14c., "big, clumsy, stupid fellow who lives in idleness," from lobre, earlier lobi "lazy lout," probably of Scandinavian origin (compare Swedish dialectal lubber "a plump, lazy fellow"). But OED suggests a possible connection with Old French lobeor "swindler, parasite," with sense altered by association with lob (n.) in the "bumpkin" sense. Sometimes also Lubbard (1580s), with pejorative suffix -ard.
Since 16c. mainly a sailors' word for those inept or inexperienced at sea (as in landlubber), but earliest attested use is of lazy monks (abbey-lubber). Compare also provincial English lubberwort, name of the mythical herb that produces laziness (1540s), Lubberland "imaginary land of plenty without work" (1590s).