A SORE tooth is enough to put anyone in a bad mood. A sore Tusk, however, is a temperamental event on an altogether bigger scale.
Tusk is usually a cool character. So while many observers were swept up in the reaction to his remarks, others asked themselves what had brought them about and what the outburst tells us about the pressure on the Brussels high command as Britain's March 29 leaving date looms.
The trail, fascinatingly, leads back to a plot to stop Brexit spearheaded by none other than former PM Tony Blair.
EU insiders tell me that Tusk allowed himself to become convinced Brexit was going to be overturned after a plan was constructed between senior Brussels figures and British pro-Remain politicians, led by Blair, that seemed to be working perfectly.
Part of the plan involved Brussels playing hardball in negotiations, so that the deal Theresa May was able to put in front of MPs was profoundly unappetising.
That advice to Brussels was relayed by a stream of pro-Remain visitors from the heart of the British establishment.
One of them, the philosopher AC Grayling, was even caught on camera telling the European Parliament's Brexit chief Guy Verhofstadt: "What would help the Remain movement in the UK is if the EU is very, very tough and uncompromising on a deal."
And so it came to pass, with the EU offering Britain almost no concessions but us being tied to the interminable "Irish Backstop" with no unilateral right of escape.
Indeed, when the Withdrawal Agreement negotiated by Theresa May was defeated by 230 votes in the House of Commons in mid-January, Tusk dropped a public hint that he now thought Britain would stay in the EU, declaring: "If a deal is impossible and no one wants no-deal, then who will finally have the courage to say what the only positive solution is?"
For his part, Mr Blair has been a fierce opponent of Brexit from the off, campaigning against it in the referendum and ever since.
But it was last year that he stepped up his efforts, insisting he believed he could prevent it. He publicly stated in November: "Up to the end I am going to do everything I can to stop it."
In an interview the previous month he gave a clue as to how: by using the pro-Remain majority in the Commons to defeat the Government and secure a postponement in our leaving date.
This in turn would be used to fight and win a second referendum.
"If you do get to a blockage in Parliament that is what opens up the possibility of going back to the people," said Blair.
It was all going so swimmingly. The trap was meant to spring shut on Brexiteers in a series of Commons votes on January 29.
The key one was an amendment put forward by Labour's Yvette Cooper - a one-time protegé of Blair's - and the Tory MP Nick Boles.
It sought to outlaw leaving the EU with no deal and put back the target date for Brexit to the end of the year, leaving time for the Blairite "People's Vote" campaign to force a second referendum in the autumn.
Given the 230-vote thrashing May had suffered a fortnight earlier, the Remainers were confident of success.
But that was not how it panned out. Instead a motion was passed backing the Withdrawal Agreement provided the Irish backstop was removed.
And the Cooper amendment was defeated by 23 votes, with enough Labour MPs from pro-Leave seats voting against it to counteract the pro-Brussels Tories who supported it.
That result was devastating for Brussels and for Tusk in particular. He had been parroting a line that the UK Parliament must stop saying what it didn't want and start saying what it did want, confident that May would fail to assemble a majority for anything.
But suddenly she had. And the bid to create space for a second referendum was in ruins.
So now the Brussels high command is in a state of disarray. It had come to believe that playing hardball would lead to the blocking of Brexit.
Now it fears the result will instead be the UK leaving on WTO terms on March 29.
That would reduce the EU's access to its biggest export market just as several EU countries are on the verge of recession, put in jeopardy most of a scheduled £39billion UK divorce payment and leave the Republic of Ireland facing a very difficult time.
If Mrs May can exploit this new dynamic and hold her nerve then there is a very good chance that the Irish backstop will indeed have to be dumped and a more equitable leaving deal agreed.
And there is no doubt in my mind whom Mr Tusk is really angriest with.
Not Brexiteers, but the has-beens of the Remain campaign - Blair chief among them - who led him up the garden path.
As he noted coldly in that same press conference on Wednesday, "there is no political force and no effective leadership" for a bid to keep the UK in the EU.
Translation: Mr Blair may think he is a messiah, but to the bigwigs of Brussels he is now just a very naughty boy.
