[a man's coat] characterised by a knee-length skirt (often cut just above the knee) all around the base, popular during the Victorian and Edwardian periods.
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You can see that he is dressed in a long black coat which was called a frock coat, and a queer, shiny hat shaped like a stovepipe, which was called a top hat. This was the uniform of the capitalists,
George Orwell. 1984 (1949)
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PODKOLYOSSIN. What do you say?
STEPAN. I say, he's started on the buttonholes.
PODKOLYOSSIN. Well, didn't he ask what your master wanted a frock-coat for?
STEPAN. No, he didn't ask.
PODKOLYOSSIN. Maybe he asked if your master was about to get married, eh?
STEPAN. No, he didn't say anything.
Gogol, Nikolai. The Gamblers and Marriage
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a portly upright man (whom I can see now, as I write) in a well-worn olive-colored frock-coat, with a peculiar pallor overspreading the red in his complexion, and eyes that went wandering about when he tried to fix them,
Charles Dickens. Great Expectations (1861)
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On the right side of the warm church, in the crowd of frock coats and white ties,