How bad is a liver punch?

The only first and last I did that (I was then 16), my former neighborhood bully was struggling to breath, in pain and had to be assisted.

I'm now in my 20's and still kind of shock that just one good liver punch from me (a 5'5 thin girl) was enough to get that reaction. I thought it was an exaggeration at first. Hence I'm asking how bad is that?
How bad is a liver punch?
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