“I'm behind you on this one hundred percent.”
You have a problem at work with people from the sales department missing deadlines for the reports they have supposed to give you. You want to change the policy to make the deadlines earlier and to punish who misses it. You discus this plan with the director of the sales department, he agrees and he says this.
I'm behind you on this one hundred percent.
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This means to agree with someone and support their plan or decision. When you tell someone that you are "behind" them, it means that you agree with their opinions but doesn't necessarily mean that you will use your time or money to help them.
You don't have to include "on ___" with this phrase if it's obvious. If someone tells you their plan and you agree with it, you can say:
I'm totally behind you.
- I'm 100% in agreement.
- We're 100% committed to making this partnership work.
- They're on board 100%.