Gangrel

“He is only a wretched gangrel creature, but I have him under my care for a while.” (Frodo, speaking of Gollum to Faramir : LotR iv. iv)

The choice of this unusual word is doubly appropriate. The first sense in the OED is 'a vagabond' and, when used as an adjective, ‘vagrant': Was it a light thing that gangrel thieves should burn and waste in Mid-mark and depart unhurt, that ye stand here with clean blades and cold bodies?
(William Morris The House of the Wolfings, chapter xxii)

The second sense is 'a lanky loose-jointed person', which may seem equally apt for the agile Gollum. The word is a Middle English formation which adds a disparaging suffix (also seen in mongrel and wastrel) to a stem apparently meaning 'go or walk'.

Tolkien also used the word (but only in the first sense) in Feanor's dismissal of Melkor: “Get thee from my gate, thou gangrel” ('Annals of Anian' 97)

The Ring of Words ~
Tolkien and the Oxford English Dictionary
Oxford University Press (2006)