[gjm11]

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.