A collie is a medium-sized dog with a long coat, kept as a family pet or trained to move sheep. The Rough Collie is the long-haired “Lassie” look; the Border Collie is the quick, staring sheepdog many people have seen in videos. It is an ordinary animal word, not slang and not formal. Though actually, not every sheepdog is a collie — plenty of farm dogs do that job without the name — and a collie is not a collier, which is a coal miner, a different word that only looks similar. The name may come from an old link with coal, because some of these dogs were dark, but that is a “perhaps”, not a settled story.