Pauper is an old-fashioned word for someone who is poor — really poor, like the paupers described by Charles Dickens or Mark Twain.
The noun pauper has been around for over 500 years, but today, the word tends to mostly crop up in literature. If people use it in conversation, they tend to use the word self-deprecatingly. "I'm sorry, I can't afford to go with you to the movies. I'm living like a pauper this week, until I get paid."