Wiktionary
vb. (context idiomatic English) To start to become worse; to worsen.
Usage examples of "take a turn for the worse".
Without that beautiful geologist, would his life take a turn for the worse?
He had seen more battles take a turn for the worse through the most simple, straight-forward aspect of the plan going awry.
Harkat appeared to have the advantage of his foe, but Mr Crepsley was moving into place to back him up should the battle take a turn for the worse.
A day and a half of nearly continual hard riding had made his disposition definitely take a turn for the worse.
If things do take a turn for the worse, you have my personal guarantee that your contracts will be `terminated with cause' from our end, and you will be free to leave.
If things do take a turn for the worse, you have my personal guarantee that your contracts will be ‘.
Apparently, the Freedom Alliance would like to see those relations take a turn for the worse, and it also seems likely they calculated that we'd be less likely to launch some sort of military operation on one of the Association's member planets.