From the Oxford English Dictionary
Deathly as an adverb: Resembling death ("deathly cold").
Hallows as a verb: To make holy or set aside for God ("Bring their candles to be blessed and hallowed").
My interpretation? a blessing ceremony that turns deadly or looks like it does -- which doesn't work because it is too overtly religious.
Deathly as an adjective: Causing death, resembling death or pertaining to death ("deathly mists," "deathly stillness" and "His wounds, many and deathly").
Hallows as a noun: A saint (All Hallows=All Saints), a saint's shrine or collection of relics (can also be pagan relics). Note: The noun is usually plural.
My interpretation? a powerful place or object that appears death-like or causes death.
So I suspect that this has something to do with graves (as others have noted, hallowed ground), either those of the founders, or of Harry's parents. It could also refer to a place where relics of the founders have been collected. I don't think the title was meant to be overtly religious.
Could "Deathly Hallows" refer to the veil? Jo told us last year that
"The veil's been there as long as the Ministry of Magic has been there, and the Ministry of Magic has been there, not as long as Hogwarts, but a long time. We’re talking hundreds of years. It's not particularly important to know exactly when, but centuries, definitely." (Source: Leaky Cauldron/Mugglenet interview, 2005)We have never learned the official Ministry name for it, so it is possible it could be called the Deathly Hallows.
Why not use 'deadly?' 'Deathly' is more poetic and old-fashioned than 'deadly.' It could be that she thought 'deadly' was too melodramatic sounding, or the Hallows could be an old place with an old-fashioned name. I also think she was researching uses of the word 'death' when she chose the name for her October Wizard of the Month: Lorcan d'Eath.
Hallow? Hollow? Jo probably expects us to confuse hallow with hollow. They sound alike, and she has deliberately used sound-alikes before (Kreacher, Diagon Alley, etc.) to flavor our mental images. Remember, in her 10/31 diary entry she said "I've now got a third title. [...] Title three currently ahead by a short nose, or perhaps that should be a vowel and two consonants." I'll bet one of her earlier titles had hollow in it, not hallows. Etymologically, they *are* connected as old-fashioned ways to shout hello, especially while hunting. (Thanks to NanB, a Lexicon reader, for reminding me of the diary entry).
Arthurian Hallows? There is so much speculation on the Leaky Lounge and the Lexicon about the 13 Arthurian Hallows that I have been reading scholarly articles about the Arthurian/Irish hallows all night. Maybe the articles have made me grumpy, but I think that though this may be the kernel of what gave Jo the idea for a Hallows, she isn't going to adhere slavishly to the Arthurian symbolism. What am I reading?
Loomis, R.S. "The Irish Origin of the Grail Legend," Speculum, Vol. 8, No. 4. (Oct., 1933), pp. 415-431.
Wood, Juliette. "The Celtic Tarot and the Secret Tradition: A Study in Modern Legend Making,"