Green’s Dictionary of Slang

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

1985		(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?’.
2000		theStranger.com 9:23 24 Feb.–1 Mar. 🌐 You’ll get laid less, but hey, look on the bright side: You won’t wind up being anyone’s ‘accidental’ daddy.