Maundy (from Latin mandatum or possibly mendicare), or Washing of the Feet, is described in John 13:1–17 with Jesus performing this act.  Specifically, in verses 13:14–17, He instructs them:

¹⁴ “If I then, your Lord and Teacher, have washed your feet, you also ought to wash one another’s feet. ¹⁵ For I have given you an example, that you should do as I have done to you. ¹⁶ Most assuredly, I say to you, a servant is not greater than his master; nor is he who is sent greater than he who sent him. ¹⁷ If you know these things, blessed are you if you do them.”

As such, many denominations literally observe the washing of the feet on Maundy Thursday of Holy Week.  The word Maundy is thought to chiefly be derived from the Latin word mandatum for “command” or “mandate.”  The “Maundy” in “Maundy Thursday” refers to the command Jesus gave to the disciples at the Last Supper, that they should love and serve one another.

Tony Reinke writes in his article The Creator on His Knees:

Jesus knelt down to wash the disciples’ feet, a model of love for the disciples. . .

For the sandal-wearing disciples, washing feet was a common cultural practice. It was proper hospitality to offer your guests a basin of water for their feet. But guests were usually expected to wash their own feet. Washing the dirt off someone else’s feet was a task reserved for only the lowest ranking Gentile servants, and Jewish slaves were often exempted from this duty. In a household without slaves, everyone washed his or her own feet.

Yet Jesus willingly dropped to his knees in the position of this extra-lowly slave to wash the disciples’ feet. The disciples were immediately shocked, and it seems, embarrassed by this act of humility. But their surprise should be no surprise to us. “There is no instance in either Jewish or Greco-Roman sources of a superior washing the feet of an inferior.”² And this was the Creator of the universe on his knees washing the dirt from the callused feet of his followers!

When Simon Peter refused to have his feet washed, Jesus said, “What I am doing you do not understand now, but afterward you will understand” (John 13:7). Whatever the meaning of the foot washing, it was not immediately evident to the disciples. The washing provided an example of love towards one another (John 13:12–17), but it also forecasted something. . . .

When we look again at Jesus’ humble act of foot washing, we see why the disciples were unable to immediately grasp the significance of the act. Jesus lowered himself into the position of a lowly slave, he served like a slave, he washed the disciples’ feet like a lowest-of-the-low slave, because ultimately he was preparing to die the dehumanizing death of a slave. In essence this is the connection made in Philippians 2:5–8.

As he washed out dirt from between the disciples’ toes, Jesus performed a parable of the cross. The disciples could not see the the symbolic anticipation, not here, not now. The full explanation for why Jesus washed their feet would only become clear after the substitutionary atonement of the Savior on Good Friday. Then they would look back and understand the act of deep humility in the cross that brought us a once-for-all, head-to-toe, cleansing from our sin.

On Maundy Thursday Jesus dropped to his knees.  He now calls us to go low in foot-washing like service to, and to be crushed for, one another.