also: あちゃー · あっちゃー · あちゃあ · あちゃ
interjection
Casual interjection used when one makes a small mistake or realizes something has gone wrong. Similar to 'oops' or 'uh-oh' in English.
あっちゃ、財布を忘れちゃった。
Oops, I forgot my wallet.
あっちゃー、またやっちゃった。
Uh-oh, I did it again.
The origin is uncertain, but it is likely a sound-symbolic expression of dismay or realization, similar to other Japanese interjections like あら or あれ.