An interesting tit bit about the release date of the title. In Harry Potter and the Order of the Phoenix, we hear parts of a prophecy during the fight in the Hall of Prophecy: "...at the solstice will come a new... and none will come after...". December 21 is the Winter Solstice. And since this is the last book, "none will come after". Coincidence? I don't think so.
Everybody has their own theories about the title. Here is mine. In this context, "Hallows" is a noun and "Deathly" is an adjective. One possible interpretation is that the title talks about "Hallows" which are deathly, i.e. caused by death.
What does hallow mean then? The POD defines "hallow" as a verb meaning "to make holy; to revere or respect". That doesn't make much sense when applied to the title. But I came across another definition of Hallows in http://www.mystical-www.co.uk/arthuriana2z/h.htm :
HallowsThe Hallows across most legends are seen to represent the royal regalia carried by the King, or the objects sought by someone such as a 'Grail Quester' in both ancient and modern stories.
This makes perfect sense if you replace "Hallow" with "Horcrux". Since the horcruxes are objects belonging to the founders( almost royalty in the wizarding world ), and Harry is seeking these objects, it all fits in. But why give a new name to something which already has a name?
The other possible interpretation is that "Deathly Hallows" is the name of a place.
Bye for now. Expect more HP&TDH related posts.