Green’s Dictionary of Slang

a top-hat.

1848		Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Plug, a hat.
1852		Lantern I 200/1: The Maine-iacs aforesaid have been in the habit of conveying such material [i.e., ‘bricks’] to their homes, secreted on their persons, generally in that article of their dress denominated the hat, castor, or ‘plug’ [DA].
1871		L.H. Bagg Four Years at Yale 46: Plug, a silk hat, of the stove-pipe or chimney-pot order. Also called beaver, tile and roof.
1883		‘Mark Twain’ Life on the Mississippi (1914) 511: ‘Captain on hurricane roof [...] kid gloves, plug tile, hair parted behind.’.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 49: plug, n. 6. A derby hat, the student’s usual dress hat. [...] 5. A silk hat.
1905		Sun. Times (Perth) 7 May 1/1: Mulga Taylor paid the penalty of siik-hatted greatness [when] the rain made a sodden wreck of the ex-prospector’s brand-new ‘plug’.
1909		‘O. Henry’ Roads of Destiny 223: He wore a high, well-kept silk hat – known as a ‘plug’ in Elmville.
1922		P.A. Rollins Cowboy 106: The Range knew that the city-dwellers wore also ‘hard’ or ‘hard-boiled’ hats, subdivided into the two classes of, first, ‘derby’ or ‘pot’ and, second, ‘plug’ or ‘stovepipe’.