don't talk nonsense; stop talking rubbish
A humorous, idiomatic retort meaning 'That's nonsense; you're talking in your sleep.' Literally: 'Say your sleep talk after you go to sleep.' Used to dismiss absurd or unrealistic statements.
"If I win the lottery, I'll quit my job." "Don't talk nonsense."
That impossible plan is just nonsense.
A set phrase combining 寝言 (sleep talking) with the imperative 寝て言え (say it after sleeping), literally meaning 'say your sleep talk after you go to sleep.' The exact origin is uncertain, but it is a traditional humorous retort.