expression
no matter what; never; under any circumstances
Used with a negative predicate to strongly emphasize that something will not happen or should not be done, equivalent to 'absolutely not' or 'under no circumstances'.
間違っても彼にそのことは言わないでください。
Whatever you do, please don't tell him about that.
間違ってもこの書類をなくしてはいけない。
Under no circumstances should you lose this document.