A phrase originating from US forces slang during the Vietnam war, where the tour of duty was fixed so the ‘grunts’ knew exactly how long, to the day, until they were due back in ‘the world’. A short timer was one who didn’t have long to go and therefore didn’t want to put himself at undue risk — hence “I’m too short for this shit”.
Another popular reference to this expression is “I’m too old for this shit”, a catchphrase for Danny Glover’s character in the Lethal Weapon series of movies.
Terry adds:
“‘I’m too short for this shit’ is a line that has appeared in at least two grunt movies. I had intended Cuddy to use it in the sewers…”