Imps come from Germanic folklore. They're lesser demons, but they didn't used to be exclusively evil. Instead they were more like fairies--tricky, mischievous, fond of pranks, but not always cruel. They could control fire, and often tried to befriend humans. They'd still play jokes on any people they managed to charm, though. Hence the word "impish."
Imps show up in all sorts of books, including the Bartimaeus books by Jonathan Stroud (which I love!). Whee for imps!