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."
When a man goes out / leaves a party with a person he does not find attractive, due to unspecified reasons, but ends up sleeping with anyway.
A man who can't control his sexual urges once the opportunity presents itself.
Person 1: Hey, what happened after you left the party?
Person 2: I ended up going with that girl to her place and we had some intimate times.
Person 1: I thought you weren't into her.
Person 2: I got a slippy dick man, I can't control it.