Desert Camo Chemical Protective Trousers! This is vintage military, dated March 1991. The size says -
So I'm not entirely sure what that equates to. It has little suspenders than tie on the front and Velcros to fit snuggly at the ankle. The waist is also Velcro so it can adjust a bit.
These come in a vacuum sealed bag that they came in originally, I am not selling the one on display.