Why do we dog lovers throw the word MUTT around like a swear word?

-these are the sweet little or big dogs that we all encourage people to go to the shelter and adopt. is there not a nicer way to refer to them even if we don't encourage the procreation of them? i know it's an acronym for something but can't remember what it is. to call dogs mutts just has a negative connotation even if they are of combined qualities and intelligence and are wonderful companions, we still try to pick them apart and guess at the heritage. how about a new acronym like Most Underrated Tail Toters? somebody think of a better one than that.