The phrase "dumb shit" is an informal way of describing something or someone as foolish, foolishness, or doing something silly or stupid. Full definition
I don't want to see a whole bunch of humans doing dumb shit. (fatmovieguy.com)
People that have no logical argument to what I'm saying automatically feel the need to make their self look like a dumb ass by saying dumb shit. (bgr.com)
Long starts telling me about the other dumb shit he does. (sbnation.com)