1. 撞击声;拍打声
    A thwack is a sound made when two solid objects hit each other hard.

thwack

    e.g. I listened to the thwack of the metal balls...
           我听着金属球相撞的声音。
    e.g. Then the woodcutter let his axe fly — Thwack! Everyone heard it.
           然后那个伐木工脱手甩出了斧头。哐！每个人都听见了。

1. a hard blow with a flat object

1. deliver a hard blow to

    e.g. The teacher smacked the student who had misbehaved

    Synonym: smack