This is taken from the movie Bill and Ted's Bogus journey. It refers to evil robot Ted and evil robot Bill seeing the photos of the princesess and Evil Ted getting a semi-erection and saying "I got a full-on robot chubby." Chubby being his penis before becoming turgid.
It has nothin to do with being turned on by robots. It is to do with ROBOTS being turned on!
Evil robot Ted: I got a full on robot chubby.
Evil robot Bill: Excellent! (Probably!)
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.