Green’s Dictionary of Slang

southern can n.

[SE southern, thus south adj. (1) + can n.1 (1c)]

(US black) the buttocks.

[US]W. McTell ‘Southern Can Is Mine’ 🎵 You get crooked your southern can belong to me / Ain’t no need you bringing no jive to me / ’Cause your southern can is mine.
[US](con. 1930s–50s) C. Major Juba to Jive.