It's been discussed recently, but I can't find the thread.
Essentially, it's a term used by some instructors to describe a condition where the engine is producing more than its design/certified thrust.
For example, our 744 isstructor used the term to tell us the EECs on the CF6 engines protected against overspeed and overboost (overthrust?) in the normal mode, but overspeed only in the alternate mode.