It's rude to wear someone else's underwear without their knowledge.
Adam is getting dressed and realizes he has no underwear clean when he see his brother's.
Adam wants clean underwear.
Adam does a load of laundry and makes sure the load contains underwear.
Adam takes his brother's clean underwear and wears them for the day.
Adam's brother wears his clean underwear the next day.
Adam's brother is offended that Adam used his underwear without his permission.