What alias did Frodo use in “The Lord of the Rings”?

The answer is: Underhill

When Gandalf sends Frodo off with literally the most dangerous object in Middle-earth, with nobody but his gardener for backup, he suggests using an alias to throw the Ring Wraiths off his trail. "Underhill" seems like a pretty clever choice at first - it's a real Hobbit family name that exists in both the Shire and Bree, so it wouldn't raise suspicion.

The plan kind of backfires at the Prancing Pony when some actual Underhills from Bree hear about the new "Mr. Underhill" and start asking questions about this possible distant relative. Some nosy Bree-landers even try to figure out exactly where in the Shire his branch of the family lives.

When "The Fellowship of the Ring" was being filmed, Peter Jackson's adaptation might have had a simpler explanation - the Baggins family home, Bag End, was located in the village of Underhill, which, true to its name, sat beneath one of the Shire's many hills. Sometimes the obvious answer is right there under your nose... or in this case, under the hill.

In both versions, the Ring Wraiths eventually catch on and everyone drops the alias. But if you ask me, Frodo Baggins sounds way better than Frodo Underhill anyway.