look up any word, like fleek:
 
1.
"Dick In Mouth" or "D.I.M" is an acronym for someone(usually a guy) whos is basically an idiot, acts like a retard or is simply a dumb fuck and would look better with the "dick in the their mouth"
"Hey dick in mouth , what the fuck are you doing over there? You gonna get that done or pound salt up your ass?!"
by Optimus Doozer October 20, 2006
 
2.
Pretty self explanatory. When something sucks or blows or is just dumb. Can be used as a play on words.
Example 1:
Arianna: You're an asshole.
Robbie: What?
Arianna: You're an asshole.
Robbie: What?
Arianna: You're an asshole.
Robbie: O i couldn't here you on account of Dick in Mouth .

Examlpe 2:

Teacher: Today we are going to talk about our feelings.
Student: That's so Dick in Mouth .

Example 3:
Fred: When i was little, i fell down the steps and knocked out all my teeth and that's why i talk with a lisp.
Brianna: O thats def D.I.M.
by Big Sammy Fish Head May 16, 2008