A dictionary is a function that takes a word and returns a set of definitions. Those definitions are composed of other words, so unless you know the meaning of those words, you won't know the meaning of the original word.
What if you find the original word in one of the definitions for one of the words in the word's definition? How deep does this recursion happen?
How much of the existing words do you need to know before you fully understand an arbitrary word? If you search with no limit, do you exhaust all the words?