They should have some kind of "definition" challenge - if you don't know the definition of a word, you can't use it.
The trouble is nailing it down. You can't expect people to remember the dictionary definition word-for-word, but then how close is "close enough", and who decided? OTOH, if you just require people to use the word in a sentence, even ruling out variants like "I want to play the word "quux" next", all a player would have to do would be to remember that a word is, e.g. a noun, and say "I saw a xyzzy on the way to work yesterday."
The trouble is nailing it down. You can't expect people to remember the dictionary definition word-for-word, but then how close is "close enough", and who decided? OTOH, if you just require people to use the word in a sentence, even ruling out variants like "I want to play the word "quux" next", all a player would have to do would be to remember that a word is, e.g. a noun, and say "I saw a xyzzy on the way to work yesterday."