Simple phrase used to catch out British spies or agents when masquerading as the enemy. When wished 'Good luck' in English by a foreign agent they find it literally impossible not to reply with 'Thanks old bean!' or something similar.
Most famously used in The Great Escape.
Hans Nazihausen: Halt! Sprechen sie Englisch?
John Smithson: Nein! Der English sind Schwein-poopen.
Hans: Haha, Ja. Good luck.
John: Thanks old bean!
Hans: AHA!
John: Oh bother.
Not to be confused with good lick, this phrase bids a person with positive farewell and hopes they do well in any situation they may or may not have gotten themselves into.