What is gnac?

I'm not sure about the spelling. In english phonetics it is pronounced 
"nyack"

At work there is a guy who often makes mistakes. This leds to big losses
on 
the factory production line.
When some of the others complained to management about him the reply was:
"Oui, mais il est volontaire. Il a le gnac. Et c'est ca q'on chreche ici"

So what is "gnac"? I'm in the Charente, between Angouleme and Limoges. 
perhaps it is a local colloquialism?

Ta
Stew