bent
mod. dishonest; crooked.I'm afraid that Paul is a little bent. He cheats on his taxes.A lot of those officeholders get bent while in office—if they weren't before.
dirty deal
n. an unfair deal.That was a dirty deal. I feel cheated.I got a dirty deal at that shop, and I won't go back.
fly-by-night
mod. undependable; dishonest.Bruno seems like such a fly-bynight character.He's not fly-by-night at all.Don't do business with fly-by-night people.
monkey business and funny business
n. silliness; dishonest tricks.That's enough monkey business. Now, settle down.Stop the funny business. This is serious!