Hundred year old egg
From Discworld MUD Wiki
These can be bought from Disembowel-Meself-Honourably Dibhala in Bes Pelargic. Eating one will make you ill, giving you a gradually increasing penalty to dexterity, from -1 to -8.
They are restricted items in Bes Pelargic, much like weapons, so if an imperial guard sees you with one it will be confiscated.