Green’s Dictionary of Slang

where the sun doesn’t shine phr.

also where the skin turns pink

(US) a euph. for the anus.

[US]P. Conroy Great Santini (1977) 9: I’ve seen him take a drink or two to feed that wild hair that grows up there where the sun don’t shine. [Ibid.] 83: ‘Kiss me where the skin turns pink,’ Bull bellowed.
[US]B. Gifford Night People 179: You can stick the Egyptians where the sun don’t shine, far as I’m concerned.
[UK]Guardian 28 Feb. [Internet] Hence we head into areas of prolific cursing, actual soiling of trousers (and on-camera inspection) and insertion of toy cars where the sun doesn’t shine.