colloquial contraction of 〜てしまう, indicating completion (often with a nuance of regret, finality, or emphasis): 'to do completely' / 'to end up doing'.
宿題を忘れちゃうところだった。
I almost forgot my homework.
このケーキ、全部食べちゃうよ。
I'll eat all of this cake (completely).
その仕事は今日中に終わらせちゃうつもりだ。
I intend to finish that work completely by today.
also used (as contraction of 〜てしまう) to express accidental or unintended actions: 'to do accidentally' or 'to end up doing (without meaning to)'.
スマホを落としちゃった。画面が割れた。
I dropped my phone by accident. The screen cracked.
話している途中で言い過ぎちゃったと気付いた。
I realized halfway through the conversation that I had said too much by accident.