Underhill Family
From Tolkien Gateway
Underhill was a common family name amongst Hobbits.
The Underhill Family had branches in the Shire and in Bree. In particular, there was a family of Bree-hobbits who lived in Staddle (on the east side of Bree-hill) who had that name.
When Frodo went incognito to Rivendell he took the name "Mr. Underhill" whilst in Bree - the Bree-hobbits thought it likely that Frodo was some long-lost cousin of theirs.
