you never know
COMMON You say you never know to mean that it is not certain what will happen in the future, and to suggest that there is some hope that things will turn out well. You never know, I might get lucky. There might be an even bigger one — I doubt it, but you never know.
you never know you can never be certain; it's impossible to predict. informal
you never ˈknow (with somebody/something) you cannot be sure (about the behaviour of somebody/something or the quality of something): Don’t throw away those old stamps. You never know, they might be valuable one day. ♢ You never know with him; one day he’s smiling and laughing, the next day he won’t speak to anybody.