John 13:4,5
got up from the evening meal and laid aside his outer garments. And taking a towel, he wrapped it around his waist.
After that he put water into a basin and started to wash the feet of the disciples and to dry them off with the towel that was wrapped around him.
The WT "translates" himatia as outer garments but some other translations says just clothes.
If he were wearing just an underwear, why wrap a towel around the waist? Usually one just wrap a towel around the waist when naked.
Even if he were in underwear it's a very strange situation...