Unputdownable is an adjective used to describe something, especially a book, so riveting that it is difficult to put down. If you’ve ever forgone chores, work, or homework because you were busy reading a really good book, that book was unputdownable. We also find numerous instances, all in Indian sources, of the word used to mean not able to be defeated.
Unputdownable is a new word, but not so new that dictionaries haven’t noticed it. Merriam-Webster says its first known use was in 1947, and the earliest example in the Oxford English Dictionary is also from that year. But a historical Google Books search uncovers some earlier examples, including one from an 1842 edition of the London Medical Gazette