out of the blue; all of a sudden; unexpectedly
Idiomatic expression describing something that happens abruptly and without any prior indication, often with a nuance of surprise or inappropriateness. Literally 'like a stick from a thicket'.
Out of the blue, he announced he was resigning.
I don't know what to say when you ask me something like that all of a sudden.
Literally 'like a stick from a thicket', suggesting something that appears suddenly and unexpectedly, as if a stick were thrust out from a bush. The exact origin is uncertain, but it is a well-established idiomatic expression.