Because spendthrift contains the word thrift, which on its own means frugal or wise with money, some people mistakenly assume a spendthrift is someone who is frugal or careful with money. But it actually means the opposite; a spendthrift is someone who spends money recklessly or wastefully. In Middle English, where spendthrift originates, thrift meant well-being or prosperity, so spendthrift literally translates in modern English to spend prosperity. In other words, a spendthrift is someone who spends what he or she has.
Frugal Germans initially balked at bailing out what they see as spendthrift Greeks. [Christian Science Monitor]
He scoffed at the notion that Google would become a spendthrift now that it’s thriving again. [AP]
But the manager himself has promised a policy of prudence rather than a return to the club’s former spendthrift ways. [Sunday Sun]