This phrase was used in the Vietnam war. The Vietnamese would camouflage themselves and hide in the trees and attack the Americans (which we 100% deserved.) the Americans were not used to this type of guerilla war fare and were very alarmed. They would yell “they’re in the trees” to warn the rest of their squad about the attack from above. Now this is a phrase that angsty teens say to express a sudden attack, or they just yell it out the window at unsuspecting pedestrians.
by Plumviolet April 25, 2019