What does the expression "dressed like a slob" mean? Where does it come from?
A slob is, to put it charitably, a person who aspires to have the energy to be merely untidy some day. It derives from the Irish Gaelic slab, meaning mud. It connotes laziness, carelessness and boorishness.
One wouldn't expect, say, a crisply ironed shirt or razor-sharp creases in a slob's clothing. Mismatched socks, pizza stains on a socially inappropriate tee shirt, rumpled and ill-fitting trousers, et cetera, would better describe the uniform of the true slob, although "dressed like a slob" is a relative phrase -- it means something different in the suit-and-tie world than it does at the ballpark.
A slob is a lazy, slovenly or untidy person. You won't expect a slob to be dressed stylishly.
From the Online Etymology Dictionary:
1780, "mud, muddy land," from Ir. slab "mud," 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 1861.
The earliest reference I found of "dressed like a slob" is from 1919's The Century illustrated monthly magazine (volume 99):
Her face flamed, and she glided over to her mother, sobbing out, "Oy, Mama, I 'm dressed like a slob!" Mirke, who happened to be busy with a peasant customer, was startled out of her wits.