Arse is a slightly rude word for bottom. Americans use the term ass.
Either way, a cow's arse is quite big and to hit one with a banjo should not be too difficult - not that we would, of course!
So if a striker couldn't hit a cow's arse with a banjo he is usually terrible at shooting and misses even easy opportunities. To be fair, some of these guys can play but even great players have terrible misses.
That's good news for the rest of us who aren't perfect either.
