Definition from Wiktionary, the free dictionary
grass; straw; draft (of a document); careless; rough; manuscript; hasty
|tree; wood||all; each and every; in all cases||
soldiers; a force; an army; weapons; arms; military; warlike
|simp. and trad.
Literally: (to be so scared and paranoid, that when the wind blows the) grass and trees, (one thinks that) the (enemy) soldiers are everywhere (and are about to attack)
- describes a situation in which someone is being overly paranoid