1 [intransitive always + adverb/preposition, transitive]
to move or make something move from side to side in a violent or uncontrolled way
The girl was thrashing about in the water.
Salmon thrash their tails and leap from the water.
to beat someone violently, especially in order to punish them:
My poor brother used to get thrashed for all kinds of minor offences.
3 [transitive] informal
to defeat someone very easily in a game:
Brazil thrashed Italy 5-0.
thrash something ↔ outphrasal verb
We still have to get together and thrash out the details.