Underhill is a very common Hobbit family name in both the Shire and Bree. Underhill Village is also the name of a section of Hobbiton that is under Hobbiton Hill whence the Underhill family derived their namesake. Underhill was the name used by Frodo Baggins during his travels when he did not wish to be identified-- an example of this being his time at The Prancing Pony in Bree. When some Underhills in Bree think Frodo is an Underhill, they believe him to be one of their long lost cousins. Gandalf told Frodo that he should use the name Underhill on his travels, to be safe.