TLA for Return To Unit.
Normally a term that is found applied to squaddies who have been thrown of a course in disgrace for major or minor infractions of the rules.
It should be noted that a fair majority of RTUs can be linked to minor transgressions linked the soldiers favourites of alcohol, sex and breaking petty rules, often all three together : such as drink beer on course, shag foxy chick in corridor of female accommodation under sign saying Out of Bounds to all Male Personnel.
It should be noted that on a more serious note, solders can be RTU'd for more serious reasons such as injury or compassionate grounds.
Captain Sir Ranulph Twisleton-Wykeham-Fiennes OBE, 3rd Baronet was RTU'd for attempting to blow up Castle Combe with purloined explosives. This was actually a deniable Black Ops mission sanctioned by persons unknown to rid the world of the threat from the evil Dr. Dolittle and his insidious experiments designed to make poor fluffy animals "talk". It made THEM very unhappy, so they waved the magic wand and transformed him back into a Donkey Walloper.