Here's how the etymology works: thrive: prosper thrift (~= having-thrived): prosperity, hence money thrifty: acting so as to get or maintain thrift, hence no

Here's how the etymology works: thrive: prosper thrift (~= having-thrived): prosperity, hence money thrifty: acting so as to get or maintain thrift, hence not spending much thrift (derived meaning): the quality that thrifty people have; again, not spending much spendthrift: a person who spends his or her thrift; hence, someone who spends a lot spendthrift (derived meaning): the quality that spendthrifts have; that is, spending a lot.