A "heater" is a slang word for a pistol or a hand gun. Dallas Winston, the toughest and most street-wise member of the greasers in The Outsiders, grew up on the streets of New York. He has been arrested many times (he brags about his police file), and he is more prone to violence than the Curtis brothers or the other gang members. He carries a heater in case he should ever need it, and Johnny knows this; Dally loans it to Johnny and Pony when they go on the lamb.
In both the popular literature of the 1950s and 1960s and in the films, especially so-called film noir detective stories, a “heater” refers to a firearm, specifically, to handguns. Detectives and gangsters would refer to themselves or to others as “carrying heat,” or “packing heat,” meaning armed with a handgun. In S.E. Hinton’s novel The Outsiders, that word occurs several times in discussions of weaponry, especially in Chapter Five, when Dallas is ‘schooling’ the younger, more innocent Greasers, Johnny and Ponyboy, regarding self-preservation:
"I didn't tell y'all something," Dally said, finishing his third hamburger. "The Socs and us are having all-out warfare all over the city. That kid you killed had plenty of friends and all over town it's Soc against grease. We can't walk alone at all. I started carryin' a heater..."
"Dally!" I said, frightened. "You kill people with heaters!"
"Ya kill 'em with switchblades, too, don't ya, kid?" Dally said in a hard voice.
Johnny gulped. "Don't worry," Dally went on, "it ain't loaded. I ain't aimin' to get picked up for murder. But it sure does help a bluff.”
Dallas has a gun because of the "war" between the Greasers and the Socs, and death is a possibility.
I don't care