It means someone is going to give someone else a hard spanking probably using a belt or strap. The phrase was used to indicate strong punishment by an authority figure over a child.
"If I catch you drinking or gambling again, I'm gonna tan your hide, young man!"

My Pa would say 'he's gonna tan my hide' if he caught me smoking this pipe.
by Old Timer December 23, 2013
Get the I'm gonna tan your hide mug.