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.
"If I win the lottery, I'll quit my job." "Don't talk nonsense."
A set phrase combining 寝言 (sleep talk) with the imperative 言え (say), literally meaning 'say your sleep talk after you go to sleep'. The exact origin is uncertain, but it is a common colloquial expression.