A rough arsed git is normally someone that appears to have good workmanship when in fact the work they produce is to a poor standard.
That work you did looked good, until I did a propper inspection and found out your just a rough arsed git
A rough arsed git is normally and engineer on a construction site that appears to have good workmanship when in fact the work they produce is to a poor standard.
Engineer: "that wall looks good you get the weepholes in ok"
Junior engineer: "them weepholes in that wall are wrong and not where they should be, I just bodged them to make them look good"
Engineer: "you rough arsed git"