The Vietnamese word "bạt tai" is a verb that means "to box someone's ear" or "to slap someone on the ear." It is often used to describe a light, quick hit to the side of someone's head, typically used as a form of reprimand or to get someone's attention.
In more advanced contexts, "bạt tai" can be used metaphorically to describe a situation where someone is given a hard truth or a reality check. It implies that the person receives a sudden realization, similar to the shock one might feel from an unexpected slap.