peekaboo santa
The heterosexual situation wherein a man, while engaged in doggystyle sex with a woman, suddenly switches from penetrating the woman vaginally to penetrating her anally, much to the surprise of the woman. In this case, 'peekaboo' refers to the surprise element of the situation, 'santa' being a euphemism for the man's penis ('a short, jolly fat man').
'It was the best sex I'd ever had until he pulled a peekaboo santa...'