Almost everyone knows mayonnaise. Almost nobody knows where its name comes from, including the French cooks who named it.
The favorite story starts with a war. On April 18, 1756, during the Seven Years' War, the Duke of Richelieu invaded Menorca and took the port of Mahón. In the legend, his cook picked up a local egg and oil sauce and brought it to France. There it was called mahonnaise, "of Mahón." Some versions have the Duke eating it at an inn during the siege. Others say he invented it himself.
The trouble is the calendar. Etymonline finds the word first in 1803, as mayonnaise de poulet, chicken mayonnaise, and it seems to have been a new dish then. That is almost half a century after the siege. Etymonline says the late date "make[s] all this doubtful." Wikipedia dates the first appearance of the name to 1806.
The early dish was not quite today's sauce either. The first uses were always "mayonnaise of" some meat, which suggests a way of serving rather than a condiment. Etymonline describes the early sauce mayonnaise, named by 1806, as closer to a vinegar and oil dressing like aspic for chilled meat. Etymonline says that by the 1820s it had become a thickened emulsion, with egg sometimes replacing gelatin.
French cooks and critics never agreed on the name. The food critic Grimod de La Reynière said the word was not French. He preferred bayonnaise, after Bayonne, which he said produced the best hams in Europe. The chef Antonin Carême wrote magnonaise, from the verb magner. Prosper Montagné proposed moyeunaise, from an Old French word for egg yolk. Etymonline adds one more idea: Mayon is a real surname, so the sauce may honor a person.
Spain has its own claim to the sauce, if not the word. A Catalan garlic and oil sauce, all-i-oli, appears in the 14th-century Llibre de Sent Soví. A 1750 recipe by the Valencian friar Francesc Roger Gomila calls a similar sauce aioli bo. The name itself may still be a mystery.