Green’s Dictionary of Slang

1. naive, stupid, gullible.

1891		Nat. Police Gaz. (NY) 28 Feb. 3/1: Miss Smith said that French was the chumpiest kind of a chump and that she had never loved him.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 27: chumpy, adj. Foolish.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 214: chumpy, lacking judgment,. ‘I was chumpy last night.’.
1937		E. Anderson Thieves Like Us (1999) 202: You’re just a big country boy and just chumpy as hell at times.
1946		Mezzrow & Wolfe Really the Blues 93: All the puffed-up strutting little people [...] so chesty and chumpy.
2002		‘Touré’ Portable Promised Land (ms.) 156: We Words (My Favorite Things) [...] Honky. Cali. Caddy. Jiggy. Chumpy.

2. (US campus) mean, contemptible.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 27: chumpy, adj. Mean, contemptible.
1923		H.C. Witwer Fighting Blood 187: I had $5,475 in the First National Bank, a $1,800 chumpy roadster.

3. eccentric, odd.

1968

‘Sl. of Watts’ in Current Sl. III:2.