Gnomish
thlind
adjective. fine, slender
gledhrin
adjective. slender
glen(d)rin
adjective. slender
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
An adjective for “slender” in the Gnomish Lexicon of the 1910s appearing as glen(d)rin or gledhrin, both elaborations of G. glenn “thin, fine” (GL/39).