A sarcastic term to describe a score of 10 or worse on any particular golf hole. In non competitive games "A Gentleman's 10" will often be the lowest score tallied for a player even if they scored a 11, 12, 13 etc. The other players are considered "gentlemen" for not making the suffering player record his terrible score.
Bill: What did you get on the last hole?
John: It was terrible I scored a 12.
Bill: Go ahead and record it as a Gentleman's 10 and we'll call it even