On its own, the word maundy means “the ceremony of washing the feet of the poor, especially commemorating Jesus's washing of his disciples' feet on Maundy Thursday.”
Recorded around 1250–1300, the word maundy comes from the Old French mande, in turn from the Latin mandātum, which means “mandate or command.”
Gospel – John 13:1-15
1 Before the festival day of Passover, Jesus knew that his hour has come, that he should pass out of this world to the Father: having loved his own who were in the world, he loved them unto the end.
2 And when supper was done, (the devil having now entered into the heart of Judas Iscariot, the son of Simon, to betray him,)
3 Knowing that the Father had given him all things into his hands, and that he came from God, and goes to God;
4 He rose from supper, laid aside his garments, and having taken a towel, girded himself.
5 After that, he put water into a basin, and began to wash the feet of the disciples, and to wipe them with the towel wherewith he was girded.
6 He came to Simon Peter. And Peter said to him: Lord, are you going to wash my feet?
7 Jesus answered, and said to him: What I do you know not now; but you shall know thereafter.
8 Peter said to him: You shall never wash my feet. Jesus answered him: If I wash you not, you shall have no part with me.
9 Simon Peter said to him: Lord, not only my feet, but also my hands and my head.
10 Jesus said to him: He that is washed, needs not but to wash his feet, but is clean wholly. And you are clean, but not all.
11 For he knew who he was that would betray him; therefore he said: You are not all clean.
12 Then after he had washed their feet, and taken his garments, being set down again, he said to them: Know you what I have done to you?
13 You call me Master, and Lord; and you say well, for so I am.
14 If then I being your Lord and Master, have washed your feet; you also ought to wash one another’s feet.
15 For I have given you an example, that as I have done to you, so you do also.”
Source: https://catholicreadings.org/holy-thursday-readings/?expand_article=1