The word "unfit" is correct and can be used in written English.
You can use the word "unfit" in a sentence to describe a person or thing that does not meet the necessary standards or qualifications for a particular purpose. For example, "The damaged bridge was deemed unfit for public use after a thorough safety inspection".
In a further statement, the Duggan family said the report "confirms their belief that the IPCC are unfit for purpose".
Owen Paterson has shown that he's unfit for office.
But if you were thinking that five-a-side is just an excuse for unfit middle-aged people to have a quick kickabout before adjourning to the pub, then an editorial in this month's British Journal of Sports Medicine brings interesting news.
Andreas Lubitz, 27, had hidden a sicknote declaring him unfit to work on the day of the disaster before boarding the Düsseldorf-bound Airbus A320 and flying it into a mountain in the southern French Alps, killing all 150 people on board.
"It's utterly irresponsible that he flew even though he had a certificate saying he was not ready to work, and was therefore unfit to fly.
At this stage, the carefully calibrated plans for English votes for English laws, in which there is a veto for English MPs over legislation purely affecting them, will look unfit for purpose to many rightwingers.
Hadn't the 1978-9 "winter of discontent" branded Labour as unfit to govern, barring them from office for 18 years?
This has led to more deaths as traffickers have exploited the situation using boats that are unfit to make the crossing," the statement said.
"Labour voters say Miliband unfit to be PM," trilled the Sunday Times.
Describing a New Year's Eve bar brawl in which several people were seriously injured and some of the attackers were reportedly Roma, Zsolt Bayer said "a significant part of the Roma are unfit for co-existence.
The one piece of good news for Sutton came before the kick-off when Rosario, the lanky Norwich striker they feared, was declared unfit.
