Extremely deeply-penetrating sexual intercourse (vaginal, anal, oral, etc.), measured by multiplying the length of ones' index finger. (See formula below).
Formula: If your index finger is 3 1/2 inches long, then to go "five deep" would equal 17 1/2 inches of penetration.
Lx5=P where L=Length of index finger and P=Penetration depth
Franco: "Hey, what's your sister up to tonight?"
Jared: "Why do you ask?"
Franco: "I was hoping to go five deep tonight, and she's the only bitch I know who's seasoned enough for that kind of punishment!"
Jared: "I'll kill you".