mug's game
A task or activity that is foolish, dangerous, or will not result in a positive outcome. Primarily heard in UK. Allowing your children to play with fireworks is a mug's game. They are bound to get injured.
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.
1992 Economist From the way many western businessmen talk, you would think investing in eastern Germany was a mug's game.