Super / really — the Kansai booster that went national.
Swap it in for とても or すごく and everything sounds younger and warmer: めっちゃ美味しい, めっちゃ疲れた. Still faintly Kansai-flavoured, which is part of the appeal — it sounds friendly.
Kansai dialect, from めちゃくちゃ (chaotic, excessive). Osaka exported it to the whole country.
Casual. In formal speech or writing you want とても or 非常に instead.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Japanese — it uses めっちゃ the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Japanese.
Free to start. No signup for your first conversation.