The next Harry Potter comes out this weekend, and the title of the book includes a word, "hallows," that is most often used as far as I can tell in one of two places.
1) The Lord's Prayer, where we pray, Hallowed be thy name.
2) Recitations of the Gettysburg Address, by Abraham Lincoln
The word means, in both cases, to sanctify or make holy. You could say, "Holy be your name", for example.
Hallow is also part of a larger word often used in our culture- Halloween, or All-hallows-eve. Halloween is the night before All Saints Day, the day when churches remember the saints. Since the saints are those who have been sanctified, or made holy, in Jesus Christ, they are "hallowed."
I imagine that J.K. Rowlings has more of the pop culture version of Halloween in mind with the title she has used. She's also smart enough that the term probably has a lot of resonances in the book. I'm not sure what a "deathly" hallows is. A halloween where deaths occur. An unholy hallows? The opposite of a place that is sanctified or holy? We'll all have to wait until this weekend to find out, but in the meantime, my definition will have to do.