Green’s Dictionary of Slang

accidental daddy n.

also a.d.
[ironic use of SE]

a man who fathers a child by accident, usu. after a casual sexual encounter; esp. in abbr.

(con. 1979) T.E. Murray ‘Lang. of Singles Bars’ AS 18: A.D. n.phr. Accidental daddy (usually used only by men and in combination with BC ‘birth control’, as in AD or BC? ‘Are you using birth control, or am I risking a paternity suit?’.
[US] 9:23 24 Feb.–1 Mar. [Internet] You’ll get laid less, but hey, look on the bright side: You won’t wind up being anyone’s ‘accidental’ daddy.