To gainsay is to deny or contradict something. Nobody can gainsay it, gainsay the evidence, hard to gainsay.
It is used almost entirely in the negative — there is no gainsaying it, nobody could gainsay him — which is a strong way of saying the point is unanswerable. The gain- is an old prefix meaning against, and it survives in this word alone.
1verbgainsayTo deny or contradict something.
“The witness was so clear and confident that the defence could not gainsay his account.”
gainsay is a denying.
Middle English gainsayen, from an old prefix gain-, against, plus say. The prefix is related to again.
Middle English
gainsayen
English, today
gainsay
Built from
From Middle English gainsayen, ȝeinseggen (“to say against, say in opposition to”), equivalent to gain + say. Compare gmq-oda gensige (“to speak against; gainsay”), Swedish gensaga (“a speaking against; protest”).
Etymology from Wiktionary, CC BY-SA