“She didn't even break a sweat.”
You played tennis against your mother-in-law. She's a really good player, so she beat you easily. Now you're telling your father-in-law what happened.
She didn't even break a sweat.
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(someone did) not break a sweat
This means that a person did something without sweating.
I used to be able to run five miles without breaking a sweat. These days, though, I get tired a lot more easily.
It can also mean doing something easily. For example:
She can already solve complex math problems in her head without breaking a sweat.