These words refer to the extravagant or imprudent spending or use of one’s resources. Spendthrift is the most informal of these words and the most clear and specific. It points exclusively to the spending of one’s money in ways that are excessive, unwise or unnecessary: putting their children on strict allowances to cure them of being spendthrift . The word by itself carries no implications about the money available to such a spender or about the consequences of such spending: a legendary spendthrift millionaire; the penurious rich and the spendthrift poor. The word was once, perhaps, more disapproving than it necessarily is now: uncomplaining when he was hard up, but spendthrift with his windfalls.

Of the remaining words, prodigal is closest in meaning to spendthrift , though it refers beyond the spending of money to any lavish or foolish extravagance, often one of awesome proportions: a prodigal shopping spree; the prodigal extravaganzas that Hollywood turns out. The word may be used without disapproval for extreme generosity: prodigal in the time she spent with any student who needed extra help. In any case, the word emphasizes the quantity of expenditure involved, whereas spendthrift can conceivably suggest the unconcerned spending of what one has, regardless of the amount. Sometimes, prodigal is understood as referring to someone who, after having wasted all his substance, returns home in repentance, after the Biblical parable of the prodigal son.

Improvident , the most formal of these words, indicates the unwise use of anything, with a specific emphasis on failing to foresee or provide for the future: the improvident grasshopper in the fable who, unlike the industrious ant, had failed to lay in supplies against the winter; improvident bon vivants who live only for the moment, with no thought for tomorrow. Thriftless relates to improvident in pointing to someone unable to save money or to manage his affairs economically, but the emphasis on the future is not, necessarily present: the average bachelor’s thriftless hit-and-miss budgeting of income. The word is distinct from spendthrift in that no squandering need be implied in this case. An impoverished family could not be prodigal , though it might or might not be spendthrift with what resources it had; it would most likely be compelled to be thriftless .

Wasteful is the most general of these in applying widely beyond the context of spending money; it suggests an unwise or reckless misuse of resources that fails to get the full benefit from them: wasteful to spend so much on a dress you’ll wear only once; wasteful of the company’s times, though miserly with his own; tragically wasteful of natural resources that can never be replaced.