“Mongrel” is kind of a nasty word, isn’t it? It refers to a mixed-breed animal (primarily a dog) or plant, but it has this connotation that isn’t especially nice. I mean, we mix breeds of dogs a lot, sometimes creating whole new breeds, like a cockapoo (a cross between a cocker spaniel and a poodle). Rightfully, a cockapoo is a mongrel, but you don’t hear them called mongrels. They’re cross-breeds or hybrids. Mongrel is applied to dogs that, well, you don’t know what breeds there are, usually because no one exactly supervised the mating of the male and female dogs that went into it. We also call them mutts, which is actually a nicer word than mongrel. If you had a dog that was part Labrador Retriever, part German Shepherd, part Golden Retriever, part dachshund, etc. and someone asked you “What kind of dog is that?” you’d probably say, “Oh, he’s a mutt” rather than “Oh, he’s a mongrel,” because it sounds nicer. “Mutt” gives you the idea that he’s a nice dog, where “mongrel” implies that he isn’t a nice dog, and will probably bite you if you got too close.
You also get the sense that a mutt makes a good pet, while a mongrel doesn’t. Do you have a mutt?
from The Sound of One Hand Typing