Green’s Dictionary of Slang

1. a top hat, a silk hat.

1863		in Three Years Among Working Classes US (1865) 223: Fancy a ragged man [...] with a gun, a knapsack, a butcher’s knife and a plug hat [DA].
1866		Daily Tombstone (AZ) 16 Jan. 3/4: One of the party [...] was seen to draw a revolver from his poket and take steady aim at the plug hat.
1875		M. Thompson Hoosier Mosaics 114: He was a stout formed man of about fifty years, dressed rather seedily, and wearing a plug hat of enormous height, the crown of which was battered into the last degree of grotesqueness.
1885		Bulletin (Sydney) 30 May 6/4: Awe-struck admiration, finding vent at last in delirious jig-dancing on plug-hats, at sentences beginning with – ‘When I was speaking to the third Napoleon;’ […] ‘when I told the Czar of Russian plump and plain &c.;’ [...].
1888		A. Trumble Mott Street Poker Club 20: There was blood in Mr Gin-Sing’s eye and a brand new plug hat on his head.
1895		Truth (Sydney) 5 May 3/1: When plug-hats predominate and clean shirts prevail, the inference is that the country member has washed himself in readiness for war.
1900		Ade More Fables in Sl. (1960) 164: He was a Gentleman, and that no Cheap Skate in a Plug Hat could tell him where to Get Off.
1902		J.D. Corrothers Black Cat Club 24: He sp’iled his ketch-me-quick plug hat.
1910		F.P. Dunne Mr Dooley Says 39: See th’ lordly bachelor comin’ down th’ street, with his shiny plug hat an’ his white vest.
1911		Sun (Kalgoorlie, WA) 29 Jan.4/7: The bride looked lovely and cool [...] But we’ll bet the bridegroom didn’t, more especially if he was attired in a plug hat and a flogger.
1916		J. Lait ‘Charlie the Wolf’ in Beef, Iron and Wine (1917) 32: The son was Ned Navarre, the nifty faro dealer who could do more things with a deck of wrong cards than Herman the Great could do with a plug hat.
1924		Mail (Adelaide) 9 Aug. 17/3: Did you ever stop to think of the man who makes the magician’s trick box, his false bottom trunk, or the plug hat out of which he pulls a plant in full bloom?
1925		S. Lewis Arrowsmith 189: He’ll wear a plug hat and have a chauffeur.
1939		J. Steinbeck Grapes of Wrath (1951) 300: This fella come in with a plug hat on.
1949		Omaha World-Herald (NE) 15 May 1/5: The greenbacks picture a cigar-smoking fat man in a plug hat saying ‘In Tax Exemption We Trust’ [DA].
1952		Morn. Bulletin (Rockhampton, Qld) 30 Dec. 4/1: Nobody at the conference produced rabbits from a plug hat, or tried to sell any of his colleagues a gold brick.
1953		A.J. Liebling Honest Rainmaker (1991) 64: He always wore a plug hat and a wing collar and a cutaway coat.

2. (Aus.) a bowler hat.

1865		Mt Alexander Mail (Vic.) 30 June 2/6: A ragged man [...] mounted on a scrawny mule, without a saddle, with a gun, a knapsack, a butcher knife, and a plug hat.
1875		S. Aus. Register (Adelaide) 19 Aug. 5/3: ‘A little bit of a man, wearing a bilious-looking plug hat’ and speaking in child like tones.
1883		Nepean Times (NSW) 23 Nov. 3/3: A man about 55 years old, neatly dressed, white plug hat, kid gloves, and appearing to be a real nice man.
1892		Newcastle Morn. Herald (NSW) 4 June 9/6: King Christian of Denmark [...] goes about the streets of Copenhagen in a plug hat and isn’t even saluted by the people he meets.
1900		Dubbo Dispatch (NSW) 5 Jan. 8/2: Solid silver plate on his coffin, six plumes on the hearse, and a nigger on the box in a biled shirt and a plug hat.
1913		Truth (Perth) 6 Dec. 6/6: He was a Bond-street beau, tailored to death, and wearing the utter abomination, a plug hat.
1925		Sydney Mail 15 July 8/1: It was the time of the ‘bowler’ hat, the ‘boxer,’ the ‘hard-hitter,’ the ‘plug hat,’ the ‘derby’ — terms synonymous, but varying according to environment.
1930		Chron. (Adelaide) 18 Sept. 66/2: They are accompanied by an equally black fellow with a black plug hat instead of the usual skull-cap. He’s the master sweep and the plug hat is his badge of office.
1942		Le Courrier Australien (Sydney) 5 June 7/1: You call a bowler hat a darby or hardboiled hat: we line it up as a boxer, bocker, hardhitter, eggboiler, plug hat, peadodger, bun or hap harry.