What is the X in X-mas?
mid-14c., "big, clumsy, stupid fellow who lives in idleness," from lobre, earlier lobi "lazy lout," probably of Scandinavian origin (cf. 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. A sailors' word since 16c. (cf. landlubber), but earliest attested use is of lazy monks (cf. abbey-lubber). Cf. also lubberwort, the name of the mythical herb that produces laziness (1540s); and Lubberland "imaginary land of plenty without work" (1590s). Sometimes also Lubbard (1580s).
1520s, from lubber (n.). Related: Lubbered; lubbering.