A mug’s game
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used mostly in casual conversations, not in formal writing or speeches. 
Don't waste your breath, it's a mug's game.
Pro Tip 2/3Negative Connotation
Implies an activity is so hopeless, it's foolish to even attempt it. 
Investing in that stock now would be a mug's game.
Pro Tip 3/3British Origin
Primarily used in the UK, might be unfamiliar to English speakers from other regions. 
He reckons he can fix it, but I say it's a mug's game.