nimnode

Someone or something that is acting without common sense.

A stupid action.

In class:
*Student walks up to the white board in the middle of class and draws a smiley, then sits down without being seen*
Teacher: "WHAT NIMNODE WOULD EVEN THINK OF A STUPID IDEA LIKE THIS TO DRAW A SMILEY IN THE MIDDLE OF CLASS!!?!?!??!?!?!"