Wikipedia
FYIFV (standing for "Fuck You, I'm Fully Vested") or FYIV is a piece of early Microsoft jargon that has become an urban legend: the claim that employees whose stock options were fully vested (that is, could be exercised) would occasionally wear T-shirts or buttons with the initials "FYIFV" to indicate they were sufficiently financially independent to give their honest opinions and leave any time they wished.
In internal usage at Microsoft, it was meant metaphorically to describe intransigent co-workers. In press usage and popular culture, it was used to imply a predatory business culture reaching even to the programmers.
Despite many third-hand reports of Microsoft employees wearing "FYIFV" buttons or shirts, there is only one report of an actual "FYIFV" T-shirt, worn on the wearer's last day at the company.