1. When, in sexual foreplay or sex, a man sneaks his penis between a woman's breasts without first getting consent for a titfuck.
2. The way most girls are introduced to the activity of titfucking.
Guy 1: "Get any boob action from that apple at the pool party last night?"
Guy 2: "Yeah. She was just going to give me a handjob but I managed to slip it in under her bikini top for a surprise titfuck. Felt good, man."