Little do they know

[mikko4]

Could anyone explain to me this phrase: little do they know.
As far as I know it means that "they know almost nothing", but I may be wrong.

Is this phrase used just in formal speech or also in informal?
Can you give an example.

Thanks in advance.

 

[David L.]

In this phrase, 'little' means 'hardly or not at all', so yes, it means 'they had no idea' (that something was going to happen/was happening).

You will see it mainly used in stories, where the author will write something like:
"Later that afternoon, Harold's watch stopped working while he was waiting for the bus. Harold reset his watch to a time given by a bystander. Little did he know, that this simple, seemingly innocuous act, would result in his imminent death."
(adapted from the movie, "Little Did He Know")