A Tokyo TV presenter, born in Yokohama, educated in Tokyo, employed by a Tokyo network, tastes something on camera and says: "めっちゃ美味しい！"
Nobody blinks. Nobody notes that this is a regional dialect word from 500 km away.
Osaka noticed. Osaka has been noticing for thirty years.
Standard Japanese has とても (totemo) and 非常に (hijou ni) for "very". Both are correct and neither is alive. とても美味しい is what a textbook says. めっちゃ美味しい is what a person says.
The word spread out of Kansai through comedy — Osaka comedians dominate Japanese television, and language follows the funniest people in the room. By the 2000s it was standard youth speech nationwide, and today plenty of young Tokyoites do not register it as dialect at all.
There is a whole family: めっちゃ (very), めちゃくちゃ (ridiculously, also "a total mess"), めちゃんこ (older, cuter, regional). Intensity roughly increases with syllables, as it should.
For a learner it is a cheat code. Attach it to any adjective and you sound conversational immediately: めっちゃ眠い, めっちゃ寒い, めっちゃええやん. It is the fastest available upgrade from textbook to human.
Japan looks like a country where the capital sets the standard and everyone follows. But the words people actually feel things in — めっちゃ, しんどい, ちゃう, アホ — keep coming up the map from Osaka, carried by comedians, while Tokyo supplies the polite forms for work. One city runs the paperwork; the other runs the emotions.