How do you decide what a word means?

I find a lot of debates often can be reduced to semantic disagreements, so it’s crucial to establish a firm definition for the terms you’re using.

Do you go by what the dictionary says, no matter what? Does it matter which dictionary? I think this could be problematic, since dictionaries can be sort of lousy at capturing connotation and nuance.

What if the dictionary had a definition that was outdated, and a more common colloquial meaning had emerged? Could you use the most popular meaning per Google? Is popularity of use sufficient to confer meaning? Would you poll experts on language, or experts in the field from which the word was derived? Or something else more obvious that I’m completely missing?