/ ˌgeɪnˈseɪ; NAmE / verb
( gain·says / -ˈsez; NAmE /, gain·said , gain·said / -ˈsed; NAmE /) [ vn ] ( formal ) (often used in negative sentences) to say that sth is not true; to disagree with or deny sth
SYN deny :
Nobody can gainsay his claims.
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WORD ORIGIN
Middle English : from obsolete gain- against + say .