a) In US Army slang, to describe something or someone vitally important to an enterprise. Said to come from the mythical Jesus nut that, supposedly, holds on a helicopter’s rotors.
b) In US Army slang, as a term of contempt for someone with an over-inflated opinion of themselves.
c) An overenthusiastic Christian.
Yeah! He knows it's the goddam Jesus Nut when we're on patrol tomorrow.
b) Look at that STRAC REMF struttin' round. Who's he think he is, Jesus Nut to the whole goddam war?
c) That padre is a real Jesus Nut!