The Deathly Hallows refer to three magical objects, which when possessed by one wizard and used correctly, provide immense power, including protection from death. They are described in the Tale of the Three Brothers, one of the children’s stories contained in the book The Tales of Beedle the Bard.
According to the story, Death becomes angry when three brothers thwart his plan for their demise by creating a bridge over a treacherous river. Death pretends to congratulate them, and offers each a prize for their ingenuity. The first brother asks for a “wand more powerful than any in existence”, the second asks for “the power to recall others from Death”, and the third, for “something that would enable him to go forth from that place [the bridge and river] without being followed by Death”. Death grants each wish, giving the first a magic wand made from an elder tree, the second a stone that would bring back the dead, and the third, a Cloak of Invisibility.
Naturally, the brothers discover their gifts are in fact too dangerous. The first brother duels with his Elder Wand, wins, but is then murdered in his sleep. The second uses his stone to summon back from the dead the girl he wanted to marry, but she is ghostly and suffered, so he ended up killing himself to join her. The third, however, was wiser, and used his cloak for many years, lived a full life, and had a family. He eventually was ready to die, removed the Cloak of Invisibility and bequeathed it to his son, and joined Death as a friend.
This tale is familiar in many ways. Though original in important aspects, it recalls the legendary journeys of the Knights Templar as depicted in Indiana Jones and the Last Crusade, as well as the quest of Utnapishtam, an ancient Sumerian hero who sought immortality from the gods. The defeat of Death, or at least an attempt, has been the plot of many recent fantasy novels, too, including To Ride a Pale Horse by Piers Anthony, Reaper Man by Terry Pratchett, and the Philip Pullman series His Dark Materials.
So what about the name itself, the Deathly Hallows? The first word, deathly, obvious means of or pertaining to death. It is not the same as “deadly”, which refers to someone or something that can cause death. We talk about a deadly assassin or a deadly disease, or in the wizarding world, a deadly potion or deadly curse. But deathly is a much less common word, used in phrases like deathly pallor.
What then is a “hallows”? The word may sound familiar because of the holiday Halloween, also known as All Hallow Eve(n). Halloween is actually a modern contraction, or as linguists say, a corruption of All Hallow Eve. Hallow itself means to “make holy” or “consecrate”, or “honor as holy”. A long time ago a hallow was also a saint or holy person, but that usage is archaic. The word comes from Old English hālgian (a verb), from the noun hālga, and is related to the word holy.
The word hallow lingers in English in a few other forms, most famously the phrase “hallowed ground” to refer to a sacred or consecrated area, as well as phrases like a “hallowed family tradition”, though there seem to be few of these left nowadays.
In the book, however, the Hallows are objects, which is a new usage for the word. Once again Rowling has extended the English language in her own quest to create a epic fantasy tale. The three objects of power, the wand, stone, and cloak, would certainly be revered and respected, in other words, hallowed. Thus, calling them Hallows makes sense, and given what they are and can do, their name, The Deathly Hallows, is entirely appropriate.