adj. 25B6; adjective (informal)
the windscreen's a bit iffy, but it's a good car : SUBSTANDARD, second-rate, low-grade, low-quality; doubtful, dubious, questionable; informal not up to much; Brit. informal dodgy, ropy.
that date is a bit iffy : TENTATIVE, undecided, unsettled, unsure, unresolved, in doubt; informal up in the air.