thrash, thresh

Thrash or thrash means to beat severely, literally or figuratively, as in an athletic competition--
Dad says he's going to thrash us when we get home.
The team hopes to thrash its rival on Saturday.
Thrash also means to toss and turn about wildly, as if making beating motions--
If he starts to thrash about again, call the doctor.

Thresh or thresh means separating grain or seeds from stalks by beating or flailing--
After the harvest we will thresh the grain.
We use a machine to thresh the grain--it only takes a day or two now.

Now that you know that, you can say--
"Our boss threatened to thrash us if we don't thresh all of this grain by the end of the day."

Note--not to be confused with "trash", which may mean refuse or garbage--
Take out the trash on your way out.
Trash may also mean, informally, to make an awful mess and break things--
That gang of juvenile delinquents sure did trash this place.
Trash may also mean, informally, to spread gossip or rumors about a particular person--
All he ever does is trash his co-workers; no one wants to listen to him.