The word you're groping for is arse I believe. Frankly, you should know that as you seem rather accomplished at talking out of it; as your namesake says to Cleon in _Knights_ pt12;
"Ah! noble Inspector of Arses, let me congratulate you."
Nice to see my classical education actually coming in useful for a change.
SS Awesome! I will use that line with my next spelling or "Aristophanes" mistake.