An abbreviation meaning "Rapid Onset Terminal Stupidity" is when a person dies or is badly hurt due to the consequences of doing something insanely stupid.
They aren't just terminally stupid, they're being stupid in a way that's going to get them killed quickly.
Either directly by their own action, or indirectly by doing something so stupid someone else cuts to the chase for them.
"Hey did you mean about what happened to Marcus. I heard he got shot putting his piece in down his jeans with the safety off."
Phrase told by Evelyn Harper in Season 9Episode 9 of Two And Half Men. Alan is expelled from Walden's house and goes to his mother's house. She denies him and says that as farewell.