to not know; to be unaware of something; to not be clear about

Only God or heaven knows; used to say that something is impossible to know, understand, or explain.

Baidu Knows; a popular Chinese online question-and-answer platform operated by the search engine [[百度|百度|bai3 du4]]

Used to indicate that one has understood or accepted a piece of information, similar to "OK" or "I see".

Who knows; used to express that something is unknown or was completely unexpected, often meaning "who would have thought" or "God knows."

Used to express that something was unexpected; how could one have known?

used to express regret or hindsight about something that has already happened, often followed by a statement of what the speaker would have done differently