The original meaning was ‘the game is over – all is lost’. More recently it has come to be used to mean ‘ we have seen through your tricks – your deceit is exposed’.

From Shakespeare’s Cymbeline, 1611:

Euriphile, Thou wast their nurse; they took thee for their mother,
And every day do honour to her grave:
Myself, Belarius, that am Morgan call’d,
They take for natural father. The game is up.