Green’s Dictionary of Slang

popo n.

(US) the buttocks.

1952		W. Winchell On Broadway 21 Aug. [synd. col.] Ho. Ho. Hit him on the Po-Po.
1958		H. Ellison Web of the City (1983) 119: She had a round little po-po on her.
1972		B. Rodgers Queens’ Vernacular.
1989–2003		R.O. Scott Gay Sl. Dict. 🌐.

In phrases

pound someone’s popo (v.)

(US gay) to sodomize.

1972

B. Rodgers Queens’ Vernacular 88: anal intercourse [...] pound one’s popo (kwn SF, late ’60s: ‘Admit it! You wouldn’t mind pounding Greg’s popo!’).