Usually an insult.
If you call someone a fanny arse and then they go look for the definition that would mean they are one.
It is usually used to illustrate that the victim is trying to win an arguement, mostly they are correct but only on technicallity of definitions and confusion.
To have a fanny for an arse (or american), to have an arse in place of a fanny, also known as front butt or double butt.
loosing person: Yeah well you ware a fanny arse
victim: What is a fanny arse? I am going on Urban Dictionary to see what one is because I should always make every arguement be 100% correct
loosing person: That's right fanny arse, you go do that...