The act of three or more males having anal sexual intercourse. Generally, the individuals stand or lie in a line, with the individual behind penetrating them.
As long as each individual serves as a link and does not break the chain, the act can be considered a "chainfuck". Therefore other forms of sexual intercourse can be performed by the end links, such as oral intercourse.
Male 1: "So what did you can the guys get up to last night?"
Male 2: "We were watching the game, but we got a bit drunk and ended up chainfucking eachother..."
Female 1: "I went to see my boyfriend last night, but I walked in on him chainfucking with his friends."
Female 2: "Really? Did you break up with him? Is he single now?"
Female 1: "Yeah, but he's probably gay."