that's DEADLY! Share definition Flag something very stupid or unpleasurable. Usually used to discribe a unfortunate event. Tron: man bro that bitch want me to take her to applebees and get the steaks !!!Will: that's DEADLY! that's DEADLY! by brad piff January 22, 2011 0 0 Get merch Get the that's DEADLY! mug. Buy Now — $32.95 See more merch