expression
don't talk nonsense; stop dreaming
A humorous retort telling someone they are talking nonsense or being unrealistic, literally 'say your sleep talk after you go to sleep'. Used in casual conversation.
「宝くじが当たったら会社を辞めるよ」「寝言は寝てから言え」
"If I win the lottery, I'll quit my job." "Don't talk nonsense."