To have sex. To "slip" implies an insertion of a body part into a cavity of another person's body. A "pep" is slang for pepperoni, which is itself an allusion to a penis due to its phallic shape.
Nick: "bruh, how did your date with Jess go?"
Josh: "it was fun man, i can see us gettin down to slip a pep in the future"
Brad: "you old dog, that's good, maybe you two can join Chad and I on a double date"
Josh: "fuck yea fuck yea bruh!"
Josh: "it was fun man, i can see us gettin down to slip a pep in the future"
Brad: "you old dog, that's good, maybe you two can join Chad and I on a double date"
Josh: "fuck yea fuck yea bruh!"
by The Great Shrimpini February 17, 2019