With the potential for nicknames Mac or Max, and alternatively spelled Maxen, this Welsh boy's name means "greatest." Macsen (MAK-sen) is from the Latin boy's name Maximus and/or Maxentius, where we get the variants Maksim, Maxim, Massimo, Maximo and Maximilian.
How did the Welsh get Macsen out of Maximus? Magnus Maximus (ca. 335 to 388) was a Roman soldier, a Christian, and made Emperor of Britannia and Gaul (thanks to his soldier buddys and a lucky agreement), controlling Britain, Africa, Spain and Gaul. He lived in Trier, the oldest city in Germany, founded around 16 B.C. His official title was Western Roman Emperor. Although Magnus Maximus was a good soldier turned pretty bad ruler whose ambition got him killed, parts of Wales can trace their heritage to him. In Wales, he was known as Macsen Wledig. An early medieval stone called the Pillar of Eliseg, on which is inscribed the name Sevira and which notes her marriage to King Vortigern, King of the Britons, could be the legacy of Magnus Maximus. There is a possibility Sevira was his daughter, as the inscription said "daughter of Maximus the king of Briton, who killed the king of the Romans," although so little has been recorded. Welsh legend claims that after Maximus married a British woman, Princess Elen, he gave her father sovereignty of the kingdom. He is often cited as founding father of the dynasties of the medieval Welsh kingdoms. In the tale The Dream of Macsen Wledig, Maximus rewards the British with the portion of Gaul to be known as Brittany.
Let's put it this way: Macsen was such an important guy that people tried their hardest not to forget about him. Unfortunately, his name remains rare in the U.S., where it has never ranked, and was given to only twelve boys in 2011, and has only been used here since 2002. The spelling Maxen was given to just fourteen boys in 2011. The spelling Maxsen was given to 5 boys in 2013.