Short answer: ‘Deviled’ is used to describe a food that is prepared spicy and probably has nothing to do with the biblical Devil.
First, a little history.
The roots of the modern-day deviled egg can be traced back to ancient Roman times, where eggs were boiled, seasoned with spices, and then typically served at the beginning of a meal.
Sometime in the early 13th century, these stuffed eggs began appearing in Andalusia (modern-day Spain). An anonymous Andalusian cookbook instructs the reader to pound boiled egg yolks with onion juice, cilantro, coriander, and pepper. After stuffing this mixture into the hollowed egg whites, the two egg halves were then peppered and fastened together with a small stick.
By the 15th century, these stuffed eggs had made their way across much of Europe. Cookbooks from medieval times contain recipes for boiled eggs that were often filled cheese, raisins, and herbs such as marjoram, mint, and parsley and then fried in oil and served hot.
The first known documented mention of ‘devil’ as a culinary term appeared in England in 1786. It was used to reference dishes prepared using spicy ingredients and/or dishes that were highly seasoned and then broiled or fried. By 1800, ‘deviling’ became a verb used to describe the process of making food spicy. In some parts of the world, the ‘deviled’ egg is referred to as a “salad egg,” “mimosa egg,” or “dressed egg,”—especially when served at church activities to avoid any association with Satan.
No one knows why the word ‘deviled’ was originally used, however, there is speculation that it was due to the Devil also being hot.