Gnomish
fenog
adjective. venomous
fenwed
adjective. venomous
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 word appearing as G. fenog or {fembrin >>} fenwed “venomous” in the Gnomish Lexicon of the 1910s, an adjectival form of G. fem “venom (of snakes), poison in general” (GL/34).
Neo-Sindarin: For purposes of Neo-Sindarin, I would update this word to ᴺS. lhoereb “venomous, poisonous” based on the later word lhoer “venom, poison(ousness)”.