I can’t find the quote I’m thinking of but I’m 99% sure it’s by Emil Cioran, and I’d understood from it that ennui is a wearied boredom caused by a person achieving a materially comfortable, predictable life. I’ll have another look later, see if I can find it.
The lead definition on Google is "listlessness and dissatisfaction arising from a lack of occupation or excitement", which fits lockdown pretty well