What does “how should i know” mean?

A sharp way of saying you do not know and that nobody could reasonably expect you to know.

How is “how should i know” used?

In The Letter (1940), one line is: “How should I know you were going to buy a gun?”

The clip at the top of this page is from The Letter (1940).

How should I know you were going to buy a gun?

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