A numbnuts is a person of 'a certain age', i.e., old enough to know better, who behaves like a total shit or shithead. A jerk, a fool, a putz, schmuck, asshole, etc.
Mike and Louise behaved like complete numbnuts over that issue. They gotmad and over-reacted, just like the numbnuts that they are.
The phenomenon that occurs when a male places a cold drink between his legs and leaves it there for a prolonged period of time. Upon standing or moving, the chilled testicles brush or rest against the leg, causing an unexpected chill to temporarily overcome the victim. Most susceptible, are wearers of boxer-style underwear.
I set my beer between my legs for a majority of the movie. When I stood up, I got numb nuts.