Trunk-top

Refers to (usually) a guy wearing either over-tight underwear or swim shorts with no shirt. This usually leads to a ‘trunk-top’ in which that person’s belly fat hangs over the top of their trunks. Similar to a muffin top, except specifically caused by swim shorts or underwear.

*Tom is wearing tight swim shorts by the beach*
Will: ‘Wow Tom, you’ve you’ve got a bit of a trunk-top going on there!’
Tom: ‘I know Will, that’s the beer belly for you!’