Green’s Dictionary of Slang

(US) splendid, wonderful.

1911		L. Pound ‘A Second Word-List From Nebraska’ in DN III:vii 543: doozy, adj. ‘Sporty,’ or ‘flossy’.
1930		G. Milburn ‘The Great American Bum’ in Hobo’s Hornbook 73: We are four bums, four doozy old bums.
1944		P. Kendall Dict. Service Sl. n.p.: doozy . . . easy, agreeable.
1960		Kerouac letter 20 June in Charters II (1999) 257: But I could really write a wild doosy book to knock out everybody.