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Related to on side: on site
on (one's) side
Supportive or in aid of one or one's idea, plan, intention, etc. We're all on your side, Jerry; we just want to see this issue resolved as quickly and easily as possible. If she's going to win this case, she'll need one heck of a legal team on her side.
on sidesupporting or part of the same team as someone else.
1997 Spectator And while clearly ‘on side’ with New Labour, he has never been a closely quartered insider.