Gnomish
pecthon
noun. plume, feather
pectha
noun. plume
cwathil
noun. plume
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.
The word G. pectha “plume” appeared in the Gnomish Lexicon of the 1910s, along with G. pecthon “plume, feather” (GL/63-64), clearly cognates of ᴱQ. pekte “plume” under the early root ᴱ√PEKE (QL/73).
Neo-Sindarin: For many years, Neo-Sindarin writers used ᴺS. pess for “feather” as the analog of word Q. quessë, which Tolkien used in the 1930s-1960s. Hialmr added this Neo-Sindarin word to the VQP (VQP) in 2015, but it was likely floating around for much longer. For “plume”, I would use an augmentative form: ᴺS. pesson “plume, (great) feather”. In 2024, a native Sindarin word S. laud “feather” was published in PE23; see that entry for discussion. Pess “feather” has been a part of Neo-Sindarin for so long that I think it is reasonable to keep using it, but if you want to stick to Tolkien’s own words, laud is probably better.