IPA: /ˈɡeɪnˌseɪər/
KK: /ˈɡeɪnˌseɪər/
A person who disagrees with or opposes something that others believe or say.
Despite the overwhelming evidence, he remained a gainsayer, refusing to accept the truth.
Gainsayer → It is formed from "gain" (from Old English *gān*, meaning to go or to obtain) and "sayer" (from Old English *secgan*, meaning to say or to speak). A gainsayer is someone who speaks against or denies something, essentially one who says the opposite of what is claimed.
Think of someone who 'goes against' ('gain') what is being 'said' ('sayer') — that's why a gainsayer is someone who speaks out against something.
No commonly confused words.