One of the most common words, but not as Gollum says it!  It’s a sound-play word which communicates both Gollum’s otherness and the echoey, huge, mysterious underground cavern.  This is one of the hisses that is the same in 1937 and 1951 – and in fact they occur in exactly the same phrase in the same numbered paragraph:

• 05.059 It’s got to ask uss a quesstion,

The 1937 version is 1937.05.059.

Updated 2016.02.11