Green’s Dictionary of Slang

sexual intercourse ‘dog-fashion’, i.e. from the rear.

1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: doggies n. Intercourse in the position adopted by dogs, usually near some dustbins.
1999		R.G. Barrett Wind & Monkey (2013) [ebook] ‘I’d heard about sex. And read about sex. And sixty-niners and doggies and things’.