Vegetarian forks were a technology developed aboard Iron Star.
They were food utensils with some rudimentary A.I. and which could emit a force-field around meat, so that a vegetarian could or could not eat said meat.
When the boys from the Dwarf came under attack from the Feral Cats, Lister and Cat attempted to defend themselves by stabbing the aggressors with forks. However when they had no effect, Lister put some lettuce over the fork so that it would be able to stab their opponents.