Like most Yiddish expressions, this is Mediaeval Low German. In Modern German it would be spelled "Ach! Gewalt!" Gewalt is a word that means strength or force, But in this context means "violence".

I see others referring to the Yiddish expression "oy vay zmir". Again, in Modern German, "Ach! Weh ist mir!", literally, "oh, woe is unto me!" In all 3 cases, the pronoun is in the Dative case, which usage has long since vanished in English, except in antique sayings such as this.

Yiddish phrase. exclamation of surprise, incredulity, or simply used to emphasize a statement. often used when kvetching. alt. spelling: oy gevaldt

A yiddish term meaning "Oh goodness!" An expression of utmost anxiety or shock.

A Yiddish expression of alarm.

Yiddish: Oh help!!!!

"Oy vey zmir"... that doesnt say much either, but its a yiddish phrase, and you can not really translate yiddish to english. The best defintion would be "oh boy or oh my gosh!"