Although it is typically used as a noun, ie "she's a swampdonkey" (referring to a particularly nasty female) it can also be used as an exclamation of immunity. If a male and his peers go to a social function, one male from the group is allowed to claim "swampdonkey". This allows said male to take home even the dirtiest of barflies home with him while his mates are not allowed to give him a hard time of any kind. This is typically used when a male is on an extended dry spell.
"Hey dude, remember that nasty chick you went home with last night?"
"Hey, shut your suckhole, dude, I called Swampdonkey".