undersign

The word "undersign" means to sign one's name at the bottom of a document, below the main text. At its core, it is a positional word: the signature is written under what has already been written. In that sense, it is close to sign, but it is much less common and usually feels formal, legal, or old-fashioned.

Picture an official letter, a contract, or a declaration. If someone undersigns it, they place their name beneath the text to show that they accept it, support it, or confirm it. That said, modern speakers usually choose the simpler verb sign in these situations. Because of that, undersign is a word you are more likely to notice in older writing or in formal fixed expressions than in everyday conversation.

This also helps explain the very common phrase the undersigned. In letters, statements, and petitions, the undersigned refers to the person or people whose names appear below the text. For many learners, that phrase is more useful than the verb itself. So if you see undersign, understand it as "sign at the bottom of a document." If you want to use natural modern English in most cases, sign will usually be the better choice.