kickkick <opponent>
Kick is an offensive warrior combat skill. It inflicts damage based on your warrior level, with bonuses for high strength and the boots you're wearing. Your character will automatically try several types of kicks, with the occasional solid mule kick doing the most damage. Kicks are not as powerful as some weapon techniques, but they can often stun opponents more quickly.
Unless your legs are restrained in some fashion or you are completely exhausted, you can always kick.
Boots with high hitroll and dexterity will help you to land kicks more reliably. Boots with high damroll will add to the damage. Boots with special damage types may even be effective against ethereal creatures.
Like most skills, you get a small amount of experience for using it.