Sentence examples for in on from inspiring English sources

"in on" is not correct in written English.
It is considered a slang expression and is not suitable for formal writing. In order to use "in on" correctly, it would need to be used as part of a larger phrase, such as "in on the joke" or "in on the secret". You can use this phrase when referring to someone who knows something that is not shared with others. Example: My siblings and I were in on a secret plan to surprise our parents for their anniversary.