Get one's due punishment or reward, as in If they put off their schoolwork to go the ball game, sooner or later they'll get theirs, or The union members were prepared to go on strike; they were determined to get theirs. The punishment version is earlier, dating from about 1900.
Be killed, as in "He'd got his. I knew it by the way his head rolled in my hands" (Rudyard Kipling, Diversity of Creatures, 1913). This usage originated in the military. [c. 1900]