In science, a dumbass (aka numbnut, numbskull, nitwit) is a type of human whose idiocy is the product of ignorance, thoughtlessness, or errors in judgment. Being a dumbass can only be validated if the resulting actions of the individual cause something to go wrong.
A common scenario among dumbasses is the annoying habit of not listening when given a set of facts or instructions. They nod in agreement while an explanation is in progress, but demonstrate lack of understanding when given the opportunity to apply the information (despite thinking they already "know it all"). This usually leads to some sort of SNAFU followed by the individual scratching their head with mouth agape.
A second common scenario is disaster due to lack of focus or attentiveness. This is why there are so many dumbass mechanics with missing fingers, dumbass programmers with buggy code, and dumbass chefs with burnt food.