To swail means to sway or swing from side to side, like a tree in the wind.
Swailing can make people think of gentle movement, like a dance or a slow song.
Examples:
The trees swail in the breeze.
I like to watch the tall grass swail in the wind.
The flowers swail back and forth.
The elephant tried to swail its trunk like a tree.
The giant swailed its arms and knocked over the buildings.
The snake swailed its body as it moved through the grass.