A spendthrift person is reckless and wasteful with his money. Spendthrifts who like to take you out to nice lunches are good people to be friends with, but it's generally a bad way to handle your own bank account.
Spendthrift was created by sticking two opposite words together: spend and thrift, which means “savings, wealth.” So a spendthrift spends all of his savings. Spendthrift people are the worst nightmare of retirement planners and Scrooges all over the globe. So unless you want to be called a spendthrift, think twice about your next purchase.