The informal British phrase (as) sick as a parrot means thoroughly dejected or disappointed (it is not known why parrot was chosen as a term of comparison—see origin).

It is the opposite of a phrase such as over the moon; and, unlike the phrase (as) sick as a dog for example, it does not refer to physical illness.

The two earliest instances of (as) sick as a parrot that I have found are from the former metropolitan county of Tyne and Wear, in north-eastern England.

The earliest instance is from Cup town dances in the streets, by Clifford Makins and Robert Chesshyre, published in The Observer (London) of Sunday 6^th May 1973; the following is the beginning of the article:

Sunderland¹, the Second Division underdogs, who started the season 250-1 outsiders for the FA Cup, yesterday defeated hot favourites Leeds 1—0 in one of the most sensational results in Cup history.
In Sunderland 1,500 shipyard workers, their wives and families hit the ceiling at the town’s Odeon cinema, where they were watching the match in colour. When a despondent Don Revie² appeared on the screen a storm of cheerful booing was followed by a splendid voice from the stalls: ‘Poor bloody Revie, he looks as sick as a parrot.’

¹ Sunderland: an industrial city and metropolitan district in the north-east of England, a port at the mouth of the River Wear
² Donald George Revie (1927-89): English footballer, manager of Leeds United from 1961 to 1974, manager of the England national football team from 1974 to 1977

The second-earliest instance is from Why Joe’s really as ‘sick as a parrot’, published in the Evening Chronicle (Newcastle upon Tyne, Tyne and Wear) of Tuesday 30^th April 1974—the fact that the author of the article punned on the phrase shows that it was already well established:

Joe Lowden is “sick as parrot” after being left holding an empty cage for the second time in a month.
The 33-year-old “Parrot Man of Percy Main³” ended up in hospital a few weeks ago after his pet Peppi flew from his home in St. Christopher’s Way and landed on house roof.
Wholesale fish merchant Joe climbed up a drainpipe to reach Peppi but he fell on to an open door and injured his pelvis.

³ Percy Main: a village in Tyne and Wear

The phrase seems to have originated in football parlance; in addition to the above-mentioned instance from The Observer, the following is from the Daily Mirror (London) of Thursday 23^rd January 1975:

          Such a choker for Lou
Manchester United ace Lou Macari said sadly after last night’s defeat at Norwich: “It’s not sour grapes, but I can’t help feeling that Norwich must be the luckiest ever to get to Wembley.
“The goal that won it for them was just like the two they snatched at Old Trafford. The referee gave a corner when it should have been a goal kick, therefore Colin Suggett should never have had the chance to score.”
United boss Tommy Docherty said: “I’m as sick as a parrot. It was ours for the taking and we had the chances after they’d scored.”

In his column published in the Daily Mirror (London) of Tuesday 15^th July 1975, titled that day Suddenly I feel as sick as a parrot, Frank McGhee even included the phrase in a list of football clichés:

Mike Channon says he wants to leave Southampton “because I want First Division football”—and it took me a very long time to work out why that statement depressed me so much.
But I finally worked out why it did—and it has nothing personally and directly to do with Channon, Southampton, Arsenal—or whoever else buys a very good player. Nor has it anything to do with England, Don Revie and the next Word Cup.
          Cheats
It is quite simply the crashing, boring realisation that the next season of Soccer is about to descend upon us once again with all its cheating, lying, hooliganism and, above all, almost total lack of communication between the private world of the men inside the game and the public, the rest of us.
Here we are about to again with:
“I want a move.”
“I’ve been made a scapegoat.”
“I want First Division football.”
“I’m as sick as a parrot.”
“They’ve only got eleven men, same as us.”
“That referee was diabolical.”
The dreary, weary round of cliches we’ve all heard before is about to start again.

Likewise, Paul Wilcox wrote the following in Bradford feel the pinch, published in The Guardian (Manchester and London) of Monday 8^th March 1976:

Lawrie McMenemy, predictably, was “over the moon” about Southampton’s reaching the FA Cup semifinals on Saturday, and if only Bobby Kennedy, Bradford City’s Manager, had declared himself “as sick as a parrot,” the great football cliche would have been complete.

 

ORIGIN: UNKNOWN

 

It has been said that the phrase (as) sick as a parrot refers to psittacosis, a contagious disease of birds, caused by chlamydiae and transmissible, especially from parrots, to human beings as a form of pneumonia.

However, no evidence supports this theory.

There is no evidence either that the phrase refers to the Dead Parrot Sketch, a sketch from the British comedy series Monty Python’s Flying Circus, first broadcast on the BBC on 7^th December 1969, in which a disgruntled customer attempts to return a dead parrot to the pet shop.

It has even been said that the phrase originated in the following quote from The false count, or, A new way to play an old game (London, 1682), a stage play by the British author Aphra Behn (1640-1689), in which Mrs. Jacinta Osborn says to her mistress, Mrs. Julia Davis, “Lord, Madam, you are as melancholy as a sick Parrot.”

But this origin is highly unlikely, since (as) sick as a parrot appeared almost four centuries after the play.

Some other ludicrous theories have been put forward; for example:

In 1909, the Tottenham Hotspur team toured Uruguay and Paraguay. On the voyage back home they were gifted the ship’s parrot by the captain of the vessel. The parrot lived happily at the club for 11 years until it died in 1919 on the very day Spurs were relegated from Division 1 and Arsenal promoted in their place.

Likewise, not a shred of evidence supports the following theory:

It is an alteration of “sick as a pierrot” and refers to the typically pale and miserable face of that French pantomime character.