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.
Example:
ex. Mike: "I have
jury duty next week."
Hank: "Haha, you said doody."
Steve: "I must do this! It is my
duty!" Ryan: "Hehe, you said doody,"