An apparatus meant to provide heat.

When playing beer pong and making two shots in a row, a.k.a "heating up"

a glock, neener, or any other type of firearm.

noun : an exceptionally attractive person; one that heats; especially: female.

A fastball in baseball lingo. It can also be used to describe a singular movement of increased intensity that is designed to bewilder another person.
Originally from the movie The Bad News Bears.

a gun, any kind of gun, due to shots going off, the gun tends to get hot, so they call it a heater caz you put it in your pants when u done

A gun.
Any kind of gun.

It's called a Heater, Because when you pop shots off, You're attracting unwanted attention from the police. As such, The situation is getting "Heaty" due to the sound coming from the "Heater"