-ちゃう or -ちゃった is in fact a contraction of the past tense of the word しまう, which means to finish or complete (-てしまった gets contracted down to -ちゃった).
It can be used for things that we did by accident or that we wish we hadn't done. The way I would put this, the way I would translate this into English, because this is the English expression that I think covers practically all cases of -ちゃった, is the word done. 忘れちゃった -- I done forgot; 猫踏んじゃった -- I done trod on the cat; スーパーヒーローになっちゃった -- I done became a superhero.
We’re saying that it happened, it’s done, it’s completed, it’s a fact, and the implication generally speaking is that we didn’t expect it to be a fact or it wasn’t what most people would normally have expected to be a fact, but it is. It “done happened”.
Không chỉ nói về "done" trong quá khứ, mà còn "done" trong tương lai, kiểu như: 夏休みが終わってしまう and that means the summer vacation will done end (Kì nghỉ hè sẽ kết thúc mất!!!!!)
It will just be and go and end. That's what'll happen and it'll be done and there's nothing we'll be able to do about it.
NOTE: One point I'd like to reinforce since I think I under-emphasized it in the video. "Done" is near-perfect for understanding ○○てしまう. In the non-past if you are ever confused, you just need to put it into the past to check. So for example (to take the future examples in the video) for 笑っちゃうだろう just put it into the past "(I told you and) you done laughed". どんどん飲んじゃう - "I done drank one after another". If it works in the past it works, in the future in the same way, even though that doesn't have a direct English equivalent. By the way, this was my 99th video, so we're coming up to the 100th next week!