It really has quite a specific meaning. And whether you side with Kierkegaard or with Heidegger, you can agree that most people use it incorrectly.
It is not merely ennui, nor is it merely anxiety. Still less is it anger. We have all those words, and we should use them when appropriate.
Angst is deeper and more existential. It is not directed outwards. You cannot have angst about your cat. Well, maybe teenagers can, but only in a stupid, hyperbolic, and illiterate way.