Meaning of “how was I to know?” in the English Dictionary

british dictionary

"how was I to know?" in British English

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how was I to know?

informal

used to say that something you did wrong was not your fault because you did not have enough information:

I just wanted to give her a surprise; how was I to know you'd already bought tickets?

(Definition of “how was I to know?” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)