Merlin's gag usually occurs after intimate pubic relations. In involves finding a rogue, ominously thick, hair at the back of your throat. Merlin's gag ensues, it's a catlike reflex, but no matter what you do that sucker is not going anywhere.
Jesus, I've had a Merlin's gag on all day! I knew I should have wet shaved her first!
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.