After his second questionable 2 letter word we added a rule (democratically voted on and adopted) that you must be able to define your word and use it properly in a sentence if anyone asks.
But that's just boring.
A scrabble word is a scrabble word. Just agree on a dictionary and be done with it.
If you really wanted to mess up your professional scrabble playing foe, you should have insisted on using a non-scrabble dictionary. Then qi and za (amoung others) wont be available and he'll be a lot easier to challenge.
Besides which, any scrabble player worth his salt will be able to give the definition that is in the scrabble dictionary
I think half the fun is out memorizing the competition.