To start, there is no reason why you should know what the word, used solely to describe this Feast Day, ‘maundy’ means. Like many ‘Church words’ it is obscure and archaic. That being said, I enjoy most obscure and archaic things, and hope you will get the same little jolt of pleasure I get when learning something odd, new, and inconsequential. 
 
The word ‘maundy’ comes from the Norman French word ‘mandé’, itself derived from the Latin word ‘mandatum’ meaning ‘commandment.’ Maundy Thursday is therefore called this because of the traditional reading read during the Liturgy on this day from John 13, ‘mandatum novum do vobis, ut diligatis invicem sicut dilexi vos ut et vos diligatis invcincem’, that is ‘A new commandment do I give to you, that you love one another as I have loved you, so you must love one another.’ 
 
Traditionally, the focus of Maundy Thursday was not the institution of the Lord’s Supper (though that has always been a part of it!) but the giving of the chief commandment of Christ to His disciples, ‘to love one another.’ 
 
Historically on this day, many churches, particularly in the Christian East, would practice a rite of foot washing, in imitation of our Lord, who washed the feet of His disciples. On Maundy Thursday, everyone enters the sanctuary barefoot, and a priest and a deaconess would seat each congregant on a chair, and wash his or her feet in a basin, the priest washing the men’s feet, the deaconess the women’s. To this day, this is still practiced by the Orthodox Churches in Egypt, Ethiopia, Syria, Iraq, and India, as well as by the Lutherans who live in those areas. In such places, the rite of foot washing itself is referred to as ‘a maundy’, as it demonstrates the clergy’s servitude to their congregation in love. 
 
So now you know, and hopefully can recall this little fact during a dinner party in the future, and impress some dear friends.