I think this term has been inhabited by me since i was little....now i use it anytime asks me how I am, even if I'm completely miserable. I don't want to be a burden on anyone else, and I know they have their own feelings to deal with, so I shrug it out, and keep it to myself.
But inside, I am not fine, but you will never know when that is, because I just just trust people enough to tell them.
I do this with my feelings too..."its OK, I understand"
I'm fine, no worries, Its, OK..one day I will let my past go.