mug
a mug's game an activity which it is stupid to engage in because it is likely to be unsuccessful or dangerous. informal
☞ Mug was mid-19th-century slang for a fool, in particular someone who has been duped by a card sharper or criminal. Mug's game appeared in the early 20th century and has been applied to a wide variety of activities, especially horse racing and betting on horses.
2012National Business Review (New Zealand) Borrowing $200 million a week for cheaper petrol is a mug's game.