Attention Surplus Disorder
As everything has its opposite and they tend to manifest on the Discworld, this is a strange and fairly rare disease first identified by the Guild of Barber-Surgeons which afflicts school pupils.
Attention Surplus Syndrome, to give it its official name, is particularly loathed by members of the Teachers' Guild, as it makes their job a lot harder. Nobody likes a child who pays attention too hard, who listens very carefully, whose eyes follow your every move, and who listens very carefully to everything you say. It has been noted that this is like talking to a great big bottomless ear.
ASS sufferers also forget nothing they are taught, and tend to offer corrections in a very audible voice in front of the whole class. If called to the front for ritual humiliation on those occasions where a sacrificial example has to be made, these are the ones who call their teacher's bluff by demonstrating they have assimilated everything they have just heard (despite the apparent fact they have been staring out of the window for the last ten minutes). The diagnostic guide advises "There is no cure. Expel at the earliest possible opportunity". The diagnostic manual is called Household Medicine, Hair-Care and Simple Surgery, and is published by Goatberger and Cropper, at $AM2.00 per copy. It is possible the Doctors' Guild has accepted that this is baby and not bath-water, and copies may be found in the Medical School at the Lady Sybil Free Hospital.
It is possible that Pteppic was suffering from a mild version of ASS when he called Mr Mericet's bluff on that memorable day (related in Pyramids) when he faultlessly listed all the things a diligent Assassin has to guard against when preparing for an active assignment. This reduced Mericet to momentary silence. Then, being a long-time classroom monster who must have encountered this before, he employed a descriptive term teacher-Assassins use for ASS in their specific educational context.
Mericet's word for the syndrome is over-confidence.
Sir Terry Pratchett revealed the origins of this concept in A Slip of the Keyboard when describing his own schooldays. Apparently he was a sufferer himself.