Once upon a time an imp was a child. Not a mischievous child, just an ordinary law-abiding young citizen. This is the kind Spenser had in mind in The Faerie Queen, when he wrote of "Fayre ympe of Phoebus and his aged bryde." Not until the 18th century did imp become both a mischievous little scut and a small, wicked spirit. Swift, in Gulliver's Travels, writes: "I once caught a young male of three years old . . . but the little imp fell a squalling, and scratching, and biting."
Thirty years later Thomas Gray used imp in a pejorative sense in The Long Story: thereabouts there lurk'd/A wicked Imp they call a Poet,/Who prowl'd the country far and near,/Bewitch'd the children of the peasants,/ Dried up the cows, and lam'd the deer,/ And suck'd the eggs, and kill'd the pheasants.
The above-mentioned imps were, in fact, used metaphorically. Whoever translated The Romaunt of the Rose in the 14th century was on the right track, etymologically speaking, when he thought of imp as a seedling: Nay, thou planted most elleswhere/Thyn ympes, if thou wolt fruyt have.
Ympe, impe in Middle English came from Old English impa, from impian. This was akin to Old High German impfon, to graft; scholars speculate that its origin was some pre-historic Teutonic word derived from an assumed Vulgar Latin word imputare, to graft, from Latin in (plus) putare to cut, prune, to cut into, so to speak. So our imp has roots in Roman soil. Margaret O'Herlihy from Ballinlough, Cork, wrote to ask about the word.
Maurice O'Brien from Limerick asks what the word libel has to do with the Latin for a book, liber. Why don't the dictionaries give more information? he complains.
The Latin liber meant the inner skin in the bark of a tree, and since Roman books were written on the inner bark of papyrus, liber came to mean a book as well, and libellus a little book. This the French borrowed as libelle, and in the 14th century the word came into English as libel. So we have Wyclif, in his translation of the Bible (1382), saying: "And the preest shal wryte in a libel thes cursid thingis."
The modern meaning of libel came into use in the 16th century. Libels were, in Tudor times, pamphlets and leaflets, often notorious for their scurrility. The law was often invoked; and soon libel gained the new sense of the publication of defamatory matter in permanent form.