Green’s Dictionary of Slang

keel n.

the buttocks.

1960

Wodehouse Jeeves in the Offing 85: He seemed to feel the rush of life along his keel.

In phrases

off one’s keel (adj.)

(US) mentally unstable, eccentric.

1945

H.B. Darrach Jr. ‘Sticktown Nocturne’ in Baltimore Sun (MD) 12 Aug. A-1/4: He [i.e. a psychiatrist] asked her if she thought she was ‘unstable’. ‘He meant was I off my keel’.