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late 13c., from Old English hlaf "portion of bread baked in a mass of definite form," from Proto-Germanic *khlaibuz (cf. Old Norse hleifr, Swedish lev, Old Frisian hlef, Old High German hleib, German Laib, Gothic hlaifs "bread, loaf"), of uncertain origin, perhaps connected to Old English hlifian "to raise higher, tower," on the notion of the bread rising as it bakes, but it is unclear whether "loaf" or "bread" is the original sense. Finnish leipä, Old Church Slavonic chlebu, Lithuanian klepas probably are Germanic loan words. Meaning "chopped meat shaped like a bread loaf" is attested from 1787.
1835, American English, back-formation from loafer (1830), which often is regarded as a variant of land loper (1795), a partial loan-translation of German Landläufer "vagabond," from Land "land" + Läufer "runner," from laufen "to run" (see leap). But OED finds this connection "not very probable." Related: Loafed; loafing.