E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The Danish thrift is the noun of the word thrive (to increase or prosper). Shakespeare says, I have a mind presages me such thrift (increase, profit). As our frugal ancestors found saving the best way to grow rich, they applied the word to frugality and careful management. A spendthrift is one who spends the thrift or saving of his father, or, as Old Adam says, the thrify hire I saved. (As You Like It.)