An imp, i, is a minor demon in NetHack. While it is not very dangerous, its fairly low AC can make it difficult for low-level players to hit. Furthermore, its speed and its ability to follow you from one level to another can make it difficult to escape. A dust-written Elbereth is good for scaring off an annoying imp.
... imps ... little creatures of two feet high that could
gambol and jump prodigiously; ...
An 'imp' is an off-shoot or cutting. Thus an 'ymp tree' was
a grafted tree, or one grown from a cutting, not from seed.
'Imp' properly means a small devil, an off-shoot of Satan,
but the distinction between goblins or bogles and imps from
hell is hard to make, and many in the Celtic countries as
well as the English Puritans regarded all fairies as devils.
The fairies of tradition often hover uneasily between the
ghostly and the diabolic state.