A phrase used as an insult from one to another.
The receiving end of the insult must be male otherwise it wouldn't make any sense and that person would automatically win the insult battle.
The exact opposite of being "hung like a horse
Guy#1: "Dude, Shut-up I'm hung like a horse
Person#1: "Yeah... hung like a seahorse!!!"
Person#1: "HAHAHAHA I win one to nothin!!!"