Okay, just checked my gargantuan, gazzillian-paged Oxford (I got up an hour earlier this morning and am now on my 3rd cup of coffee) and it gives much the same def - 'anguish' as a synonym. But origin is centuries earlier, in older ('archaic' - lovely word!) language.

re Labrat's comment that humour is something that we percieve differently:
Oh yes - ever find yourself finding a story funny that wasn't, as it turned out, intended that way? Ever been in a movie theatre and laughed at bits that no one else did? and vice versa. Think clowns - funny or The Joker in Batman 1 (and the only decent Batman movie, I might add)

c.
addit: So are there readers who sometimes find Angst Funny???