Sick As A Parrot Meaning

(idiomatic, Britain) Very disappointed; miserable.

Example: 2010 February 25, Chris Boffey, John Terry left empty-handed as Wayne Bridge gives him cold shoulder, The Guardian,
  Bridge was over the moon and Terry sick as a parrot as the game ended 4-2 following a late Chelsea penalty.
2010 December 7, Wirral Euro MP is sick as a parrot over England World Cup bid humiliation, Wirral Globe.
2011 July 29, John Walsh, Ingalls awesome but Workington Comets edged out, News & Star,
  Team manager Tony Jackson said: “I’m as sick as a parrot as I’ve ever been. There’s no doubt we should have won this one if it hadn’t been for the riders we have who suffered knocks.”