Kick

Kicking is a directional action performed by pressing Ctrl-D (or optionally #kick). Kicking has a number of uses.

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 "Thunk!".
Kicking a loadstone, even if it has room to move, will give the message "Thunk!", unless the player has the necessary strength to move an object of that weight naturally.

Kicking monsters induces (not very much) damage. This may be desired in some cases (pacifist).
Kicking a cockatrice corpse without wearing boots will result in stoning.
Kicking a sleeping monster will wake it up.

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