Green’s Dictionary of Slang

excellent, wonderful, first-rate etc.

1918		H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 28/2: KEENO. Preferred form of keen, meaning fine, excellent, beautiful, all right, etc.
1927		in G. Legman Limerick (1953) 330: There was a young fellow from Eno / Who said to his girl, ‘Now, old Beano, / Lift your skirt up in front, / And enlarge your old cunt, / For the size of this organ is keen-o.’.
1936		R. Chandler ‘Guns At Cyrano’s’ in Red Wind (1946) 214: ‘Keeno,’ Tony Acosta breathed. ‘Oh, keeno. That was Miss Adrian, wasn’t it?’.
1947		N. Algren ‘Single Exit’ in Entrapment (2009) 81: I used t’ be a keeno blonde.
1950		Goldin et al. DAUL 114/1: Keeno. See Keen.
1960		G. Swarthout Where the Boys Are 93: The keeno things were that it was under the hotel and beside the pool.
1961		F. Kohner Gidget Goes Hawaiian 48: He looked like the cleanest guy going and his physique was keeno.