Excelsior is a popular saying that Stan Lee would use to sign off of his soap boxes.
Stan Lee had several other sayings but when he discovered that Marvel's competition were starting to use the same sayings he looked for a word that they would not be able to spell or know what it means. Stan Lee eventually found the word excelsior. Excelsior is a latin term that means ever upward. Excelsior is also the moto of New York. In interviews Stan Lee has said that he would often be asked why he used the name of package stuffing in his letters. Stan Lee would then tell them to look at the second definition.