Benita Bendon Campbell passes on this wonderful bit from Cathy Guisewites’s strip Cathy:
Here we get a series of ‘penurious’ items: cost-conscious, penny-pinching, prudent, budget-minded. In the midst of these comes spendthrift, clearly intended to convey ‘penurious’, though in fact its standard use is to refer to a profligate spender.
How does this happen? Well, spendthrift has thrift in it, suggesting thrifty. ‘Thrifty in spending’ and all that. People are forever trying to figure out the meanings of multi-part words from the meanings of their parts, but that doesn’t always work. As in this case.