Taken from the old Tarzan movies in the 1960’s where all the
male characters had their
shirt backs wet with sweat. So if you have been working on a hot day and you have a wet back from sweat/workout you have Tarzan’s back.
Person 1: (Straight in to the
fridge for a beverage) Fuck its hot
outside, look at how wet the back of my
shirt is.
Person 2: Ha, looks like you have Tarzan's Back!!
Person 1: Tarzan’s back?
Person 2: You know when the bad guys were looking for Tarzan & Cheetah they all had wet shirts from the heat.