1. 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.
2. 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]
See also: get
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.