Translated it actually means "The Devil is a Part-Timer" which makes sense when you learn about the shows gag:
The Demon Lord Satan Jacob seeks to conquer the world of Ente Isla by annexing its four continents with the help of his demon generals Alciel, Lucifer, Malacoda, and Adramelech.
After being confronted by the hero Emilia Justina and her companions after they killed Malacoda and Adramelech, Satan Jacob and Alciel escape the world of Ente Isla through a gate to modern Tokyo, Japan. However, due to the lack of magic in the modern contemporary world, both Satan Jacob and Alciel change into forms representing what they would look like if they were human. In order to survive, Satan Jacob takes a part-time job in a fast food restaurant named MgRonald while Alciel serves as his houseman.
One day, Satan Jacob, who now goes by Sadao Maō, meets a girl who is actually Emilia in the form of Emi Yusa. The story then unfolds and explores the personalities of each of the characters and their moral values. More characters show up from Ente Isla and they too face the new world dilemmas, often comically.
When you actually sit down and watch the show you will see that it tends to read more like a sitcom – great play on fantasy-meets-real-world cliche, and such dry, situational humor.