Mt TBR is Goodreads shorthand for the mountain of unread books many of us own (To Be Read). (iolagoulton.com)
It could be a lul just due to having a large back log of unread books in your e-reader. (goodereader.com)
You can open the book you're currently reading or any recently read books, or browse for unread books in the file system or in any of seven bundled online catalogs — or you can add your own. (computerworld.com)