Synonyms of 'flail'
To beat on, at, or against something means to hit it hard, usually several times or continuously for a period of time.
The rain was beating on the window pains.
If someone thrashes about, or thrashes their arms or legs about, they move in a wild or violent way, often hitting against something. You can also say that someone's arms or legs thrash about.
He collapsed on the floor, thrashing his legs about.
Thesaurus for flail from the Collins English Thesaurus
If your arms or legs flail or if you flail them about, they wave about in an energetic but uncontrolled way.