I don't believe WordData will translate an ID to a word. You will likely have to use an outside source for this information.
Alternatively, WordData will translate a word into an ID:
WordData["cat", "WordNetID"]
You could build up a reversed index for a set of words that way if you wanted. Once you have the word, you can get it's definition as follows:
WordData[{"cat", "Noun", "Man"}, "Definitions"]
{{"cat", "Noun", "Man"} -> "an informal term for a youth or man"}
