Compare:
What is 'water'?
Well, in one sense 'water' is 'whatever we want it to be'. 'water' is just a word, and it is us who fix the meanings of words. If we had wanted, we could have made our language such that the word 'water' referred to chairs, or to fridges or to courage.
But there is something missing from this response. In our language as it actually stands, 'water' does not mean whatever we want it to mean. Its meaning is fixed by its role in that language. If I start calling courage 'water' people do not understand me. If I am a child they correct me, if an adult they think I have gone mad or tell me to stop drinking. In our language, it is not the case that 'water' is 'whatever we want it to be'. And the question 'what is water' is a question formulated in that language. It seems churlish to respond in a different language.
Can't we say at least these things about 'good'? 'Good' is a word in our language with a meaning fixed in that language, at least to some extent. I cannot call rape and murder 'good' whilst retaining the meaning of 'good'. If I do, people rightly correct me. At the very least, 'good' is not 'whatever we want it to be'. If it were, I cannot see what role it could have in our language. But 'good' does have such a role, and to (at the least!) this extent good is not whatever we want it to be.