which means " **** , CROUCH DOWN TO AVOID AN INCOMING PROJECTILE"
however the second man misinterpreted the statement as a joke, he assumed the first man was making a pun based on what the second man was holding, a duck.
however this was a misinterpretation as there really was a projectile coming at him, which required him to duck down, but he did not, because he made a misinterpretation, and so he was hit by the projectile
what makes this funnier is that the projectile was in fact a duck. hence the added funnyness