This expression is akin to "Woke up on the wrong side of the bed" or "Who pissed in your Cheerios?" To have a "hair" across ones ass means to be overly sensitive to irritation. Usually one who over-reacts to a situtation because there is already an issue afflicting them.
What do you have a hair across your ass about?" "Bob is usually an easy going fella but today it would seem he has a hair across his ass