A colloquial adverb meaning 'very', 'extremely', or 'really'; originally Kansai dialect but widely used in casual speech.
このケーキ、めっちゃおいしい！
This cake is super tasty!
今日めっちゃ疲れたから、早く寝る。
I'm really tired today, so I'm going to bed early.
あの映画、めっちゃおすすめだよ。
That movie is highly recommended.