1. Military term, short for 'end of exercise', heard or written on a schedule when "hostilities" during a field exercise are meant to come to an end.
2. Derived from its military meaning, used when describing something that's come to an end, or something that is complete.
Military (eg on radio):
"ENDEX, ENDEX, ENDEX."
Colloquial (eg on finishing an essay):
"That's endex then!'