This imitative interjection sounds just like what it means.  It doesn’t show up after Chapter 8.

• 05.010 Ugh! (of the icy cold water in Gollum’s lake)
• 05.051 Ugh!’ he said, (of a cold, clammy fish)
• 07.035 Ugh! here they are!’ (Beorn of Gandalf and Bilbo)
• 08.110 Ugh! (spiders responding to being cut by Sting)

“ugh, int. and n.” OED Online. Oxford University Press, March 2015. Web. 9 May 2015.

Both dwarves and river-elves cause rackets of the loud noise variety in The Hobbit.  OED gives it as possibily imitative

• 02.042 at what he called “all this dwarvish racket,”
• 09.029 Drat this dwarvish racket!’
• 09.064 and there was a merry racket down by the river.

“racket, n.2.” OED Online. Oxford University Press, March 2015. Web. 8 June 2015.

And here’s the partner-word to the one just posted.

• 04.036 croaking, jibbering and jabbering;

This word is onomatopoeic from the sound of words which carry no meaning to the listener.

• 04.036 croaking, jibbering and jabbering;

“jabber, v.” OED Online. Oxford University Press, March 2015. Web. 3 June 2015.

I am finding classes of words as I go, and I’m not sure whether to divide words more or less narrowly.  “Thag” is Tolkien’s spelling of Bilbo trying to say “thank” when he has a headcold.  It’s not onomatopoeia, but is it poetical?  It’s definitely word play.  Is it an accent, such as the trolls have (“yerself”)?  Or an idiolect, such as Gollum has (“sits with it a bitsy”)?  Is it merely a temporary mispronunciation?  Should I call these words common and not count them?  Or uncommon and include them?  I believe that Gollum, Bert, Tom, and William will have their own entries for just this purpose.  For now, I wish to include “thag” as I find it emphasizes the auditory quality of this tale.  Like sagas, The Hobbit is meant to be heard.  If we are lucky, we get to read it aloud to our children, as it was born these decades past.

• 10.039  Thag you very buch.