According to Hanks & Hodges's
A Dictionary of First Names, Nepomuk is a Czech name originally used in honor of St.
John of Nepomuk, who died in 1393. He was a priest and chaplain at the court of
King Wencelas IV, and was executed by the king, legendarily for refusing to tell the king what the queen had told St.
John in confession. Nepomuk is a town in Bohemia. Unfortunately the book does not give the etymological origin of the place name.