“It’s curtains for you Mug!!” When someone is threatened with death in the old gangster/hardboiled detective movies & stories, it was always curtains. This term derived from theatre, when the last bit of stage direction was curtain – indicating the end of the show/play, and dropping the curtain.
“It’s curtains for you” is a metaphor. The final ‘Curtain’ is drawn across the stage at the conclusion of the person’s life. The expression ‘it is curtains for you’ simply means you are finished, your escapade is over, the game is up, you have been discovered, you are about to receive your punishment, you have arrived at the conclusion of your wrongdoing, the time is up for you. The hardboiled characters used this as a threat of imminent death. So well-worn was this expression of existential threat that it became a cliché and used in parody. Even Bugs Bunny menaced those gangsters when the jigs was up.