in on something—know smth. that isn’t common knowledge:
- As Tommy, who was in on the plot, kept Maranzano busy in the office, the killers disarmed his bodyguards.
Note: The expression does not correlate in meaning with the phrase in for something—used of smb. due for a promotion, punishment, etc.:
- That gentlemen is in for a rude surprise some morning soon.
on the in with something—(U.S. coll.) well familiar with smth.:
- Any more information would be appreciated as I pride myself being on the “in” with Matthew’s films.