You are here because Tamatoa from the Disney movie Moana sang to look up the word decapod, and you searched it up for fun to see if anyone referenced the movie on Urban Dictionary. You are right.
"...You could try, try, try but you can't expect a demi-god to beat a decapod, look it up..." - Tamatoa, 'Shiny'.
Fictional slang for killing. One's map is presumably one's face, and to demap someone is to remove them from the map as well as to remove their face from among those faces you see alive. Or something.
An act of masturbation in which, while one pleasures themself, they decapitate themself with a rusty shovel or dime store katana at the verge of climax. Then, whilst their head rolls away, they attempt to catch the load in their mouth within the final three seconds of life.