What does it was anything but a break mean?

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“It was anything but…” means it’s the opposite of how it seems or should be. In this context, the word “but” is used to negate the next word in the sentence.

For example, to describe a cruel and rude person, one could say “She is anything but nice.”

In the above example, the phrase is somewhat sarcastic. As if calling her anything else in the world is more reasonable than calling her “nice.” You could call her an astronaut, a cowboy, a banana… but never nice!
;-D

“It was anything but a break” means that the break (time off from work or school) was not relaxing nor fun, like a break normally is or should be.

Dialogue Example:

“How was your summer break?”

“It was anything but a break. I worked at the factory everyday this summer.”