Kick

Kick is a directional command performed by pressing Ctrl-D (or [k] if the number_pad option is on).

Kicking has a number of uses. Kicking anything while levitating will propel you in the opposite direction.

Kicking objects

Kicking heavy or immovable objects may result in injury ("Ouch! That hurt.").
Repeated kicks may render your leg useless for a short time, making you unable to carry heavier loads ("Your leg is in no shape for kicking.").
Kicking an object that has no room to move will give the message "Thump!".
Kicking a loadstone, even if it has room to move, will give the message "Thump!", unless the player has the necessary strength to move an object of that weight naturally.
Kicking a locked door can break it down, leaving an empty space.
Kicking a locked container can break the lock, but could also break fragile items inside it.
Kicking a sink can have a number of results, including producing an ooze or a ring.
Kicking an altar can incur the wrath of the owning God.

Kicking monsters induces (not very much) damage. This may be desired in some cases (pacifist). Even an unskilled kick attack enjoys a +1 to hit bonus, so may make a desirable alternative at times when training a weapon from unskilled (-4) or with a cursed weapon welded to your hand.
Kicking a cockatrice corpse without wearing boots will result in stoning.
Kicking flying monsters can displace them to a random point adjacent to you, as they "swoop, nimbly avoiding your kick"
Kicking a sleeping monster will wake it up.

If your luck is down, you might have too much difficulty trying to search for secret doors. Sometimes you can find a secret door by kicking its location. "Crash! You kick open a secret door!" or "Crash! You uncover a secret door!" This is best when you are almost certain of the location of the door, for example when there is a corridor on one side and a room on the other.

Kicking boots will increase the strength of the kick, but at the same time might make loadstones and similarly heavy objects movable (and thus not identifiable by kicking).
Boots of any kind allow the player to kick cockatrices.