lobtail

Example:The whale thrashed its tail, causing a huge wave, and then dove beneath the surface.

Definition:To move with violent, stiff movements, especially from side to side, as if in struggle or in attempting to escape.

Example:The once elegant whale began to flail its tail, causing chaos in the surrounding waters.

Definition:To swing a part of one's body or a weapon with violent, wild, or uncontrolled motions.