A noun for “thief” in the Qenya Lexicon of the 1910s, an agental formation based on ᴱQ. naqa- “steal” (QL/64).
Early Quenya
pilu
noun. thief, robber
naqar
noun. thief
pilmo
noun. thief, robber
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.
A noun appearing as ᴱQ. pilu “thief, robber” in the Qenya Lexicon of the 1910s under the early root ᴱ√PILI “rob”, along with a longer variant pilmo (pilmu-) (QL/74).
Neo-Quenya: Since I retain ᴺQ. pil- “to steal, ✱rob” for purposes of Neo-Quenya, I would retain ᴺQ. pilu “thief, robber” as well. For example, this noun was used in Helge Fauskanger’s NQNT (NQNT). A later noun of similar meaning is Q. arpo “seizer, thief”.