Trip

Source Core Rulebook pg. 283
You can use this weapon to Trip with the Athletics skill even if you don’t have a free hand. This uses the weapon’s reach (if different from your own) and adds the weapon’s item bonus to attack rolls as an item bonus to the Athletics check. If you critically fail a check to Trip using the weapon, you can drop the weapon to take the effects of a failure instead of a critical failure.

Weapons

Aklys, Bo Staff, Fauchard, Flail, Gnome Hooked Hammer, Guisarme, Horsechopper, Kama, Kukri, Ogre Hook, Scythe, Sickle, Spiked Chain, Temple Sword, War Flail, Whip