A spendthrift is a person who spends money improvidently or wastefully, often to the point where outlay climbs well beyond means. The word serves as an adjective in the same sense.
As a modifier it is most often applied by news media to governments thought to be wasting public money, and it has one settled technical use: a spendthrift trust carries a clause barring a beneficiary from assigning their interest in advance and putting that interest beyond the reach of creditors — a legal instrument named for the person it exists to protect from himself.
The word dates to about 1600 and is built from spend plus thrift, which then meant savings, profits, or accumulated wealth rather than frugality. Thrift had entered English around 1300 from thriven, "to thrive," naming the condition of one who thrives, and only narrowed to the habit of saving in the 1550s. The older sense is what makes the compound coherent: a spendthrift is one who has spent their 📝prosperity, not one who spends their economy. The coinage displaced the earlier spend-all, attested in the 1550s, and scattergood, from the 1570s.
Profligate and prodigal run close to it. Profligate carries a stronger charge of moral dissipation, extending past money to conduct generally, while prodigal points at the lavishness of the spending rather than its consequences and is frequently misread, through the parable, as meaning wayward or returning. Spendthrift is the plainest of the three and the only one that names a person by the single act of exhausting their own resources.