Green’s Dictionary of Slang

naf n.1

also naff
[? abbr. backsl. fanny n.1 ; but note Scot. nyaph, the female genitals]

the vagina.

[US]E. Field ‘A French Crisis’ in Facetiae Americana 19: What shall I term that slimy pit-like orifice of sin, [...] quaint merkin, naf or jock?
[UK]Partridge DSUE (1984) 775: [...] from ca. 1845.