withersake
Withersake (noun) means someone who has abandoned their beliefs and betrayed others.
Etymology
Came into English from Middle English withersake, from Old English wiþersaca (“adversary, enemy; betrayer; apostate”), from Proto-West Germanic *wiþrasakō, equivalent to wither- (“against”) plus sake. It is cognate with Middle High German widersache and Modern German Widersacher (“adversary, opponent, antagonist, foe”).