In Old English impe means a young plant shoot or a tree sapling. Over the years, the word came to refer to smallish entities that were direct offspring of the Devil and sent from hell to do evil deeds to humans on Earth. Imps might well be called junior demons, and one of their principal assignments, according to Christian authorities, was to disguise themselves as black cats, owls, ravens, or some other animal and serve as a witch's familiar. In many of the transcripts of the European witchcraft trials, the demonic spirit given by the Devil to a witch to do his or her bidding was referred to as an imp.
In most of the descriptions of imps given by witches or those theologians who claimed to have exorcised the entities, their appearance apart from the animals they possessed was always a solid black in color. Since they were creatures without souls sired by the Devil, imps were condemned to be dark shadow beings, forever denied the light of God.
Over the centuries, the meaning of the word has lost its satanic implications. When one speaks of someone having an impish smile, one is likely to mean that that individual has wry sense of humor. To refer to children as "little imps" suggests that they are mischievous, rather than malignant.