Be on somebody’s side Definition 1 of 3Pro Tip 1/3Group Affiliation Implies alliance with a group during a disagreement or competition. In the team debate, he was on our side. Pro Tip 2/3Support Act Highlights the action aspect; you actively do things to assist. She raised funds to show she was on their side. Pro Tip 3/3Conditional Usage This phrase can be conditional on actions, not just beliefs. I'll be on your side if you prove you're right.