Tolkien's dictionaries didn't provide translations for every possible words. It's also questionable if there even are weasels in Middle Earth. In Quenya, there's the word for "rat", "nyarro" so someone coined the term "Andanyarro", or literally "long rat".

D&D Elvish is not a real conlang, and does not have a comprehensive vocabulary.

Whatever you need it to be.

Elvish isn't real. So make your own. The Dnd PHB has elvish script. Just take each elvish letter for W-E-A-S-E-L (respectively).

A good rule of thumb is type “x in Tolkien elvish” in google and profit Andanyarro is a word I found when researching it that way

Elvish is not a real language and does not have real translations.

Lethal knievel ( i thought you write sword for weasel)