Not wearing any underwear.
Going commando has nothing to do with shit. because uniforms are suppose to be loose and allow a lot of ease in movement and to aid in cammouflage but not too baggy to gat hung up on anything. When it is hot and humid soldiers in the field can get crotch rot and also get rubbed raw. you don't want to have to much wet scanky fabric next to that part of your body. It is because soldiers sweat a lot and they cant take showers for days. It is one the best ways way to prevent crotch rot and rashes. I should know been there done that. Former soldier infantry and medic.