heater

noun

any of various apparatus for heating, especially for heating water or the air in a room.
Electronics. the element of a vacuum tube that carries the current for heating a cathode.
Slang. a pistol, revolver, or other firearm.

noun

any device for supplying heat, such as a hot-air blower, radiator, convector, etc
US slang a pistol
electronics a conductor carrying a current that indirectly heats the cathode in some types of valve

c.1500, of persons, agent noun from heat. Of devices, from 1660s. Baseball slang meaning “fastball” is attested by 1985.