You fell down on the stairs and hurt your knee. Your girlfriend is worried, so you're reassuring her.
I just skinned it a little.
"Skinning" your knee means hurting it by ripping or rubbing some of the skin off of it. This happens when you fall down while moving fast. You can also "skin" your elbows.
Skinning your knees or elbows hurts a lot, but it's not a really serious injury.
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