Green’s Dictionary of Slang

north pole n.

the anus.

1960		J. Franklyn Dict. of Rhy. Sl.
1978		Maledicta II:1+2 (Summer/Winter) 116: The fact that the slang contains other meanings for elephant, or that au fond there are plenty of terms for ‘bottom’ – bottle (and glass), North Pole (‘hole’), and Khyber (as we have said) – deters no one.