The act of a man sneaking up behind a girl naked. The man then proceeds to pull the girls pants and undergarments down and sticking his penis in her ass all in one swift motion.
This is best done if the man can pull it off from the front sticking it in the girl's vagina, truly achieving the WTFucked. But also pissing the girl off much more.
Leon: Awe man that chick from the club last night wouldn't give me anything. So I went to the bathroom and undressed, then ran out and WTFucked her.
Kenny: From the front?
Leon: Hell yeah!