A wide trident is a two-handed spear-type weapon that is good for blocking. With a high Spear skill rating, this can be a strong defensive fighting alternative to using shields. Wide tridents do not seem to perform well when thrown.
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes the wide trident thusly:
A wide trident may have, in spite of its name, more than three points. It is mainly used in fishing, because its width makes it rather a clumsy weapon for fighting.