Primitive elvish
del
root. thick, dense
ndel
root. thick, dense
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
Early Quenya
tunga
adjective. thick, dense
A rejected root meaning “thick, dense” that appeared in rough etymological notes for the name of S. Glorfindel < glaur-phin-dela = “gold-hair-thick” (PE17/17). One of the proposed Quenya forms was nelya beside lelya, indicating Tolkien was considering strengthening the root form to √NDEL.
Neo-Eldarin: For purposes of Neo-Eldarin, I think it would be worth salvaging this root in this strengthened form ᴺ√NDEL “thick, dense”, since I know of no other good options for words meaning “dense”.