The word duty, usually defined as a task or obligation one must do, sounds very similar to the word doody, which is essentially a word for shit. So whenever someone says duty in a sentence, some will make you immature joke "
Hehe, you said doody." This is an example of a double entendre and/or type of innuendo, similar to something like when someone says something is "Long and
hard," you might laugh is response because your mind immediately thought of your dick. So instead of thinking of someones mission, your mind goes to someones
poop. Immature as hell, but still
funny.