When a man is having vaginal intercourse with a woman and accidentally slips out of the vaginal opening and into the anus of the woman, and then back into the vagina.
I had a slippy double dippy with my girl last night, but I think she liked it.
A slippy dick happens when a man has no intentions of having any intimate relations with a woman/man but ends up doing so. Mainly because the other person is not attractive enough to have sex with, but he just can't resist.
"So what happened after you left the party with that girl?"
"I got a slippy dick, you know how it is..."
"You really need to learn how to contain yourself, man."