Green’s Dictionary of Slang

a round felt hat.

1841		Punch I 98/2: Displaying to the greatest advantage those unassuming castors designated jerrys .
1847		Dublin Eve. Mail 6 Aug. 4/3: Mr Fagan [...] seized Mr Callaghan’s summer jerry. Mr Callaghan roared [...] ‘don’t break my hat, I tell you!’ .
[1859		G.A. Sala Gaslight and Daylight 113: I have seen one in a hat like a London dustman’s, in a Jerry hat [...] and in a red nightcap].
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.