Green’s Dictionary of Slang

afternoon n.

[? naut. aft, the ‘behind’ of a boat + play on SE afternoon]

(W.I., Bdos/Guyn.) the buttocks, esp. when large and female.

1970		F. Collymore Notes for Gloss. of Barbadian Dial. 10: Afternoon. A euphemism for backside.
1996		Allsopp Dict. Carib. Eng. Usage.