“Ennui” is a French loanword, but the meaning has shifted from its origin. In Old French, “enui” meant “annoyance,” and in English the usage is closer to “boredom.” Someone feeling ennui is dissatisfied with their situation and exhausted by their boredom. That feeling may be annoying, but it doesn’t quite mean “annoyance.” “Ennui” is most often used as a noun, but it also can be a transitive verb, meaning “to inflict with ennui” (essentially “to bore someone”).