Easy Question, What does this symbol mean?

BadWolf

What does the upside down hat mean in vector notation. I stumbled across this symbol in my fluid mechanics book compressible flow, Munson seems to like use lots of non standard notations to begin with but I figure this symbol is still commonly used somewhere. Hat usually implies a unit vector but upside down hat?

[tex]c_{v}=\frac{\partial\check{u}}{\partial T}[/tex]

I would guess that Munson uses the check as latex calls it to make sure you notice that he means internal energy rather than velocity as u often represents. I was curious what the history of this symbol is as this is the first time I've seen it used.