The dictionary is an attempt at systematically defining the semantic meaning of language subject to cultural constraints in the same way that a writing system is an attempt at visually rendering a spoken language subject to technological constraints. I know from personal experience that the people who work on dictionaries have a less authoritative view about them than you do.

dictionaries and common usage (e.g. wikipedia, forums, news, etc.) are certainly more authoritative than a random person unilaterally deciding that a word has no meaning and declaring "dictionaries and wikipedia are wrong"

if you poll 100 random english speakers and the majority come back saying that they have no idea what "third world" means, or say that the term is meaningless, i will concede and donate $1,000 to a charity of your choice.