in on the joke

[Maybo]

"Carpet, did you hear that?" The carpet nodded, clearly in on the joke that Aladdin wasn't getting. (Aladdin by Elizabeth Rudnick)

Why do we need "on" in the above sentence?

 

[Skrej]

'In on' is a set idiom. Don't think of them as separate prepositions, but rather a set phrase.

 

[Maybo]

'In on' is a set idiom. Don't think of them as separate prepositions, but rather a set phrase.

So does it mean the carpet understands (in on) the joke that Aladdin wasn't getting?

 

[Skrej]

Yes. The carpet knows about the details of the joke, whereas Aladdin doesn't.