The German Church traditions have wonderfully descriptive names for the days of the Triduum (Maundy Thursday, Good Friday, and the Vigil of Easter on Saturday evening).
Maundy Thursday comes from the Latin noun mandatum, which means "command" as is found in John 13,34. Compare the verse below:
»A new commandment I give to you, that you love one another: just as I have loved you, you also are to love one another« (ESV).
»mandatum novum do vobis ut diligatis invicem sicut dilexi vos ut et vos diligatis invicem« (Vulgate).
In German, however, Maundy Thursday is called Gründonnerstag (literally "Green Thursday"). This may come from the German tradition to eat green vegetables, especially spinach, on this day (Bill Petro). I think perhaps more ecclesiastically the name dates back to Medieval Europe when it was customary in the Catholic Church to give penitents the gift of green branches on Thursday of Holy Week for the imminent completion of their fasting during Lent. The term certainly isn't derived from the liturgical color for the day, as Maundy Thursday/Gründonnerstag is traditionally the color "white."
Good Friday is Karfreitag or Charfreitag in German. Grim connects the old German Karfreitag with the Old High German chara, grief, suffering, Old Saxon cara, Gothic kara (Woerterbuch, S. V.). So old English care, Anglo-Saxon cearu, mean grief. Thus, Karfreitag translates as "suffering/mourning Friday". It is also important to note that the old Teutonic cara means preparation, and links the word back to Friday as the day of preparation for the Sabbath. See Grim's reference for documentation.
Linguistics are certainly grand!
The liturgical words point us to the theme of the day, and on Maundy Thursday and Good Friday the theme is Christ and forgiveness, promised and given at Jesus last seder meal when He instituted the Lord's Supper to distribute the forgiveness of sins and purchased on the next day when Jesus offered up His very body and shed His precious blood as payment for the world's sin. And then comes blessed Easter ...