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This means someone is telling you they are going to give it to you hard, and they mean your backside. The mum part is just a joke about your mom and how you probably did something bad to her.
I'll put it your ass like I put in your mum, you little brat.
You think you're tough? I'll put it your ass like I put in your mum.
You messed with the wrong kid, I'll put it your ass like I put in your mum.