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1780, "mud, muddy land," from Irish slab "mud, mire dirt," itself probably borrowed from English slab "muddy place" (c.1600), from a Scandinavian source (cf. Icelandic slabb "sludge"). The meaning "untidy person" is first recorded 1887, from earlier expressions such as slob of a man (1861).
[1861+; fr Anglo-Irish, used affectionately of a quiet, fat, slow child]