Key is not a phone thrower, although there are occasions when he performs the non-violent equivalent, such as telling TV3's John Campbell "with all respect to your financial literacy, John, you don't have any".
But by and large Key maintains a demeanour of zen, so his "pfiss" was all there was to read his mood.
Key's "pfiss" originally only came out when he was saying something he did not necessarily mean - such as whether he believed, deep down, that it made economic sense for the retirement age to rise. In that context, it could be argued that when Key said "no, pfiss" he meant "yes".
This was good news for those whose job is to catch the Prime Minister out. The bad news is that people who are too often caught out by their tell become adept at disguising it. So Key has since sought to cover his tic's tracks by using it in all manner of situations.
It now comes out when he is getting irritated by a line of questioning and repeating himself. There is a triumphant "pfiss" when he thinks he has slain Winston Peters, as in Parliament this week when the pair performed their sequel to the riveting saga of how many elderly migrants were freeloading on NZ Super.
There is also the satisfied "pfiss" which comes out when he thinks he's made a joke. In that context, the noise appears to serve the joint function of indicating his own pride in being such a wit and a verbal order to his own backbenchers to laugh, somewhat akin to a standup comedian pulling his ear for the hired laughers strategically placed in a difficult audience.
It is this use of the pfiss that most annoys Labour, for they have long scorned Key for his stand-up style of brushing off questions and sought to paint him as the David Brent of politics - a depiction somewhat spoiled when their new leader David Shearer was caught out laughing at Key's jokes.
So Labour MPs have fallen upon this pfiss with some glee and generously started to save him the trouble of doing it in Parliament by doing it themselves - pfissing away in noisy unison whenever Key stops talking, although regrettably Hansard is yet to come up with an accepted spelling for it.
It comes out when he's not quite sure whether even he is convinced by his own spin - most recently setting out the list of 14 targets in 10 areas his Government wanted the public service to achieve - ranging from getting 20,000 people off long-term welfare to reducing cases of rheumatic fever.
Despite Key's assurance that the list of goals was "not a wish list, but a to-do list", it was much derided by the Opposition, whose most pressing concern was to find out where old lists had gone to die.
Most of those lists were devised by the Minister of Lists, Steven Joyce - such as that list which contained "goals" such as catching up with Australia by 2025, the 120-point action plan for economic growth, and the 21 point Jobs Summit plan of 2009 and National's own six-point plan for a brighter future.
The suspicion that the latest set of lists would simply wither away over time to that happy place was somewhat accentuated when even Bill English and John Key could not agree whether the lists were pipe dreams or not.
Key first of all said that some goals would be difficult to achieve and were "very aspirational". However, when asked how ministers could be held accountable for not achieving goals even the Prime Minister had described as "very aspirational", Bill English said the Government had not said they were aspirational, but that they were "achievable".
However, the only thing worse than being pfissed at is not to be pfissed at at all. Keen observers will have noticed that Key has started completely ignoring Labour and reserving his pfissing for the Green Party.
This was marked over the past week - whenever he was asked to respond to general Opposition criticisms of asset sales, he chose to target the Green Party for his counters without even mentioning Labour.
The more support in the polls grows for the left, the more Key has taken to pretending Labour does not exist.
This is all part of a ploy to consign Labour to irrelevance by giving further grist to that school of thought that the Green Party is the "real" opposition party, and therefore the only party worthy of prime ministerial rebuttals.
Key does not necessarily believe this, of course, but he is having an almighty amount of fun in feeding the perception by pretending he does.
But Labour does have its own pfiss-inducing comeback up its sleeve - it has not yet pointed out that at least it has potential coalition partners.
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