Lynne Murphy has published an interesting post on this phrase, the original version of which is with the “for.” It’s a Britishism which the OED defines as “suitable for the intended use; fully capable of performing the required task.” The dictionary has an 1861 citation but that appears to be an outlier, and the next is from a 1953 book about industrial operations: “Small-scale operation is multiplied when rival producers continually devise new designs which may or may not be fit for purpose.”
The phrase took off in Britain in the 1990s and aughts, as this Ngram Viewer chart shows.
I didn’t bother to look for it in American sources because it’s such an outlier here. The phrase has appeared in the New York Times sixty-one times, but said by British sources or written by British writers virtually always, one exception being in 2021, when the U.S. Food and Drug Administration (FDA) said Emergent BioSolutions “facilities were ‘fit for purpose and in a state of compliance.'”
But Lynne has found that Americans have taken to using an altered version of the phrase that seems to mean the same thing: “fit to purpose.” It’s a phenomenon we’ve seen before, for example in Americans changing “can’t be arsed” to “can’t be asked.”
Lynne reproduces this tweet, sent in by one of her readers:
And she says she’s found other examples of American “fit to purpose” on Twitter. It’s a trend that bears watching.