Green’s Dictionary of Slang

1. a low-crowned hat.

1852		Norfolk News 6 Nov. 3/2: Sarah Ann Moore charged [...] with having [...] stolen from his person [...] a chummy hat and a silk handkerchief.
1859		Norfolk Chron. 10 Dec. 9/3: He wore a dark ‘chummy,’ and a much worn velveteen jacket.
1859		Times (London) 7 Dec. 11/5: I could not see his face. His chummy hat shadded his face from the moonlight.
1868		Norfolk Chron. 31 Oct. 5/6: William Whistler (20) labourer was indicted for [...] stealing a watch-chain and a chummy hat.
1879		Cambridge Indep. Press25 Oct. 7/3: The defendant said he had not had a ‘chummy’. His Honor: What is a ‘chummy’? [...] Plaintiff (holding uop a wide-awake hat): This is a ‘chummy’.
1885		Cambridge Indep. Press 19 Sept. 8/1: He noticed he had some blood on his ‘chummy’ hat and waistcoat.
1894		Yorks. Eve. Post 14 Sept. 3/1: Lord Rosebery wore a ‘chummy’ hat on Tuesday, and a silk hat on Wednesday.
1907		Aus. Star (Sydney) 9 July 1/2: He was dressed in a dark tweed suit [...] and. had a light soft ‘Chummy’ hat thrown down on the seat beside him.
1908		Observer (London) 29 Mar. 8/6: Two or three wore their ordinary headgear — c cap or a chummy hat.

2. (US) a form of cloche, worn for sports.

1920		Indianapolis News 22 Oct. 13: [advert] The chummy hat — that’s the sports hat. Kt fits down snugly on your head with such a contented feeling you know it will never blow off.
1924		Honolulu Advertiser (HI) 23 Nov. 46: [advert] Chummy Hats. The Latest Sport Mode.