Mr. Underhill was the name Gandalf gave to Frodo Baggins before he set off on his quest during the War of the Ring. Sauron had already learned of Baggins through Gollum and using the name outside of the Shire would prove dangerous, thus a secret identity was formed.
This was perhaps not the best choice, for when "Mr. Underhill" arrived in Bree, there were several Underhills in the common-room at the Prancing Pony and they thought it likely that Frodo was some long-lost cousin of theirs. One of the Bree-landers, who seemed familiar with the Shire, asked where the Underhills lived and who were their relations.
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