A trick glass was a novelty item used to play practical jokes. It was almost identical in appearance to a normal glass, save several imperceptible small holes in the side. These holes allowed liquid to seep out and onto the one drinking from it.
In 2270 while the USS Enterprise senior staff was eating together, some fell victim to this prank. Leonard McCoy recalled similar pranks being played when he was in medical school. (TAS: "The Practical Joker")