It is when a man has the squirts or loose stools and needs to use some roll toilet paper like a tampoon to catch or absorb shit. It avoids a man to have shit streaks on his underwear.
Man I need to go to the bathroom to get a manpoon or my girlfriend will think that I shit myself.
equivalent to using the word poon in a sentence, it refers to the male genitalia.
Jess: That dude is hot.
Joan: I would so tap that manpoon.