On the Discworld, every quality has its equal and opposite anti-quality. For instance, there's the whole drunk - knurd thing, which can serve as illustration.
Charisma is the quality, or set of qualities, that makes certain people irresistible and casts a sort of spell which draws others into their orbit and makes them more amenable to following suggestions given by the charismatic one, and predisposing them to follow in his/her orbit like seagulls around a ship. Carrot has this in spades, quite possibly as part of his personal aura which certain excitable people have ascribed to kingship. Tawneee has a different sort of charisma that attracts men in droves to her presence; the Elves have a similar power to charm and enchant which they use for wholly selfish ends of their own.
Now consider Nobby Nobbs.
Absolutely the opposite applies to this luckless soul. Nobby quite possibly has the same amount of charisma that Rincewind has of magical ability - so low that it may even cross the zero and have a minus in front of it. This anti-aura has been described as charisn'tma. Elsewhere in the writings, Nobby has been described ironically as being a sort of sexual magnet. This was meant in jest. But held in the right way, a magnet repels every bit as strongly as it attracts, so maybe this is not so far off the mark after all...
The Canting Crew has the kind of charisn'tma that makes people pay them to go away.