He was their leader. The disciples hoped this was the beginning of a coup that would bring the Romans to their knees and usher in a new Davidic kingdom. So they were naturally shocked when he began to wash their feet, displaying the dirty business of Maundy Thursday.
It was a filthy job fit only for a slave. Yet here he was, stripped down to his undershirt and washing their dirty, ugly feet. This wasn’t the kind of coup they were expecting.

The disciples were horrified. Peter said what everyone else was thinking (He usually did). “No! You’ll never wash my feet!” He couldn’t bear to think the Messiah he looked up to would humble himself to the point of humiliation.
But Jesus made it clear that Peter would have no part in this kingdom if he refused. So he continued by washing all twelve disciples’ feet, including the one who planned to turn him in to the authorities.
When Jesus finished, he wrapped up his lesson.
“You call Me Teacher and Lord, and you say well, for so I am. If I then, your Lord and Teacher, have washed your feet, you also ought to wash one another’s feet. For I have given you an example, that you should do as I have done to you.”
He gave them a new command, or mandate, to love one another. That isn’t easy. It’s the dirty business of Maundy Thursday.