A Grandad - A grandad is when you slam your knee into someone’s thigh so

A grandad is when you slam your knee into someone’s thigh so hard, it feels like you just kicked their leg with a hammer.

I did a grandad to my brother. He walked like a grandma for a week.

At the park, I did a grandad to a kid. He cried like a baby.

I did a grandad to my friend. He now walks like a broken robot.