It's the inexplicable and uncontrollable twitch of the hands which make the afflicted - or the cursed, as they are in professional sport - look as feeble-minded and jittery as the hardened alcoholic trying to guide that last glass of tawny port to his mouth. It's the loss of fine motor skills as surely as if someone had turned off the ignition and thrown away the key. 

It's the illness we know as the yips.

In sport, that means the most regulation manoeuvre - putting in golf, serving in tennis, free-throwing in basketball, pitching in baseball, bowling in cricket, kicking a goal in football - suddenly causes a tightening of the chest, a moistening of the palms, an attack of anxiety and self-doubt.

Movements which once came so naturally - holing a metre-long putt or nailing a free throw - become a tortuous task in which the fluid is replaced by the fumbling, the second-nature by the stumbling.

Sam Snead, Bernhard Langer and many other besides have had the yips with their putting; Ian Baker-Finch was afflicted by them with his driver then, as the virus worked its way through his bag, nearly every other club he owned. Peter Alliss, the one-time pro and now veteran BBC broadcaster, remembers putting on the 11th hole at Augusta National during the US Masters in 1967 - facing a reasonably straightforward two-metre putt up the hill - and then going into a kind of trance and snapping out of it only to find his ball seven metres on the other side of the hole.

He had contracted the yips. It was a disease he was never able to get rid if, and it brought a premature end to his career. Hence his unscheduled move into the commentary booth.

The great Czech tennis player, Miloslav Mecir, had them with his serve, and was sometimes reduced to serving underarm in ATP events. Baseball history is littered with catchers who couldn't throw back to first base, and second basemen who couldn't get the ball back to first, without spraying it wide, or high into the bleachers. Ditto basketballers - such as Shaq O'Neal - who couldn't control their nerves, and their mind, long enough to hit the mark from the free-throw line.

Now, in the AFL, we have undoubtedly the most serious case of goalkicking yips seen in recent memory.

Sure, we've laughed at Matthew Richardson and Cameron Mooney and Travis Cloke and Nick Riewoldt and other forwards who've gone through shaky patches in the past decade. But North Melbourne's Lindsay Thomas has taken the problem to another level altogether.

Last year, the forward pocket won the Roos' goalkicking with a season tally of 29.26 - hardly a dead-eye dick, but a serviceable return nonetheless. This year, he's compiled 17.29, with at least a dozen shots sailing out of bounds on the full, meaning his conversion rate is now hovering somewhere below 30%.

Now, the mildly amusing sideshow has become a kind of sick joke. The yips are costing his club games - and Thomas his sanity.

Against Sydney in Round 10, the little left-footed No.12 missed a set shot in the first quarter from maybe 12 metres out. The Roos ended up losing by a point. Last week, against St Kilda, Thomas had a set shot from 40 metres out, pretty much straight in front, to put the Roos back within two points of the Saints in the final quarter. He kicked it out on the full.

Yesterday, at the MCG, in a match North never looked like winning against Collingwood, Thomas marked just 13 metres out for a rare opportunity in the third quarter. Roo fans who've watched the horror show unfold this season cannot have been overly confident, but surely the goalsneak - who's practised hundreds and hundreds of set shots at training in the past few weeks - would have no problem in slotting that through.

But, no, somehow Thomas contrived to slew that one wide as well.

At three-quarter time, ABC commentators were moved to remark on how distressed Thomas looked at the team huddle. Certainly there was a time when he was sitting on the bench with a towel over his head - and his head buried in his hands. He was a picture of pure misery.

Clearly, it's time to give him a spell in the VFL, or even a holiday back home in Adelaide. Something to clear his mind. Because there is nothing the club doctors can prescribe to cure this invisible illness, no magic pill, and he's of no use to his teammates, or coaches, in this current frame of mind.

In fact, such is his slough of despair, one wonders whether he can ever return to the sort of form that terrorised Carlton in round 12 last year - on a Friday night, in front of a national audience - when he kicked 7.5, danced around opponents, showed off all his tricks and would have won all six Brownlow votes if they were available. 

But that was before he contracted the sportsman's most dreaded affliction, the one that plays with the mind.