A formal/literary variant of 'turn on': to suddenly attack or criticize someone, or to depend entirely on something.
Suddenly attack someone who trusted you, or everything depends on one thing.
2 meanings, ordered from most common to least. Color-coded by CEFR level.
To suddenly attack or speak angrily to someone who was previously a friend or ally.
"The crowd, once supportive, turned upon the speaker when his lies were exposed."
The dog turned upon its master without warning.
— Traditional cautionary expression; widely used in 19th-century literature
To depend or hinge entirely on a particular factor or point.
"The success of the entire negotiation turned upon a single clause in the contract."
To physically rotate so as to face upon someone.
Suddenly attack someone who trusted you, or everything depends on one thing.
Largely literary or archaic. In modern English, 'turn on' is far more common in both senses. 'Turn upon' appears frequently in 19th-century fiction and formal rhetoric.
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