Green’s Dictionary of Slang

a.b.c. n.1

[like abc, it is the beginning, although of life rather than the alphabet]

the vagina.

[UK] ‘Will the Merry Weaver’ in Pepys Ballads (1987) III 132: I gave her a Fescue in her hand, And bid her use it at her command. She said you best know where it should be, Come put it to my A.B.C.
[UK]Farmer & Henley Sl. and Its Analogues.