Green’s Dictionary of Slang

(US) a man’s hat.

1908		T.A. Dorgan in Zwilling TAD Lex. (1993) 51: Hey, douse the kelly. I can’t see third [base] at all.
1919		A. Baer Two & Three 22 Apr. [synd. col.] If you didn’t have a high Kelly balanced on your 15-cent haircut, you were out.
1929		L. Thomas Woodfill of the Regulars 246: We certainly doffed our tin kellys to the Heinies for their thoroughness.
1929		A.T. Rogers ‘Casual Observer’ in Charlotte Obs. (NC) 6 Jan. sect. 3 10/8: Al Smith [...] shed the brown derby and donned a shiny silk kelly. Getting high hat, eh?
1931		Irwin Amer. Tramp and Und. Sl. 116: Kelly. – A hat, more especially a derby.
1948		Daily Ardmoreite 25 Apr. 1/1: A ring was drawn and some brawny gent who felt his strength, would toss his kelly into the circle [DA].
1951		N. Algren Chicago: City On the Make 88: A left-hander’s wind [...] rolling the summer’s last straw kelly across second into center.
1962		Ragen & Finston World’s Toughest Prison 806: kelly – A hat.
1972		N. Algren ‘The Last Carousel’ in Texas Stories (1995) 122: Sitting bolt upright [...] under a straw kelly the hue of an old hound’s tooth.