IPA: /ˈkaʊntənənsər/
KK: /ˈkaʊntənənsər/
A person who supports or approves of something, often showing encouragement or favor.
The countenancer of the new policy spoke at the meeting to express his support.
Countenancer → It is formed from "countenance" (from Old French *contenant*, meaning to contain or hold) and "-er" (meaning a person who). A countenancer is a person who supports or endorses something, literally one who holds or contains a particular attitude or expression.
Think of someone who 'holds' ('countenance') a certain attitude or expression — that's why a countenancer is a person who supports or endorses something.