let the genie out of the bottle的意思是“to allow something to happen that cannot then be stopped”，即“带来难以遏制的恶果”。

【例句】
When I told him the secret, I let the genie out of the bottle.
我把秘密告诉他的时候，便是一系列不可控恶果的开始。

Once you make carrying guns legal, you let the genie out of the bottle.
一旦你让携带枪支成为合法行为，你就带来难以遏制的恶果。