arm in arm

adverb2

1. •
   With arms linked together; with arms around each other's shoulder or waist. (of two or more people)

   • The two friends walked along arm in arm.
2. •
   (figurative) In collusion, in concert (with someone).

   • ‘Frenchmen. Really, I mean to say, Stevens, Frenchmen. […] And to think we have to be seen by the world to be arm in arm with them. One wishes for a good bath at the mere reminder.’ — 1989