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.