Gainsay, a verb, means "contradict" or "speak out against." When you challenge authority, you gainsay, as in teachers don't like it when unruly students gainsay them.

Gainsay comes from an Old English word that means "contradict" or "say against," as in, "no one dared gainsay the principal, who is well-known for giving detention to students who so much as frown at him." If you know someone who constantly corrects others, tells them that they're wrong, and says, "That's not true," more than anyone else, you have first-hand experience with the art of the gainsay.

Definitions of gainsay

v take exception to

challenge, dispute
challenge the sincerity or truthfulness of
challenge (somebody) to make good on a statement; charge with or censure for an offense
Type of:
contend, contest, repugn
make the subject of dispute, contention, or litigation

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