Someone that has no common sense and/or does not think of of the consequences of their actions.
1. Guy 1: Dude, how cool would it be if someone jumped from the top of your roof into your pool?
Guy 2: OMG, im gunna do that.
Guy 1: no way, thats like a 30 foot gap!
Guy 2: I dont care, im gunna do it!
*jumps from roof, hits legs on edge of pool and breaks legs*
Guy 1: What a retard