Green’s Dictionary of Slang

diddley-pout n.

[? diddle n.2 (2) which, when stimulated, it ‘pouts’]

the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
[US]Maledicta IV:2 (Winter) 190: Diddler is [...] that which is used to diddle (= titillate) the diddley-pout = vagina.