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.
To shove, forcefully, a fully-engorged penis into someone's rectum. Without lube and without warning. Causing a sharp rise in pulse, blood pressure, and sweat.
Guy A: "Bro, did you ever tag that Emily bitch?"
Guy B: "Please. Dude, I fucking ran 'the heater' on her."