There are two theories about the origin of the name. One is that there was a pizza shop called "Mario & Luigi's" near Nintendo of America, so they came up with the idea of ??naming it Luigi; the other is that It is said that because its characteristics are similar to Mario, and the similar Japanese word is "るいじ", so Luigi, which has a similar pronunciation, was chosen as the name.
PS: There is another theory: There is a French movie "The Reward of Terror" in 1953. The protagonist is named Mario and his partner is named Luigi, so he was named Luigi.