Undersign is a synonym for the term "sign." However, there are a few other words that can be used in place of undersign. One of these words is "endorse." To endorse something means to give approval or support for something. Another synonym for undersign is "autograph." This term is often used in reference to someone signing their name on a document or piece of memorabilia. "Inscribe" is another synonym that can be used in place of undersign. To inscribe something means to write or carve a message or name onto a surface. Overall, there are several synonyms for undersign that can be used interchangeably depending on the specific context.