The habaki (鎺) is a piece of metal encircling the base of the blade of a Japanese bladed weapon. It has the double purpose of locking the tsuba (guard) in place, and to maintain the weapon in its saya (scabbard).
A katana, a type of Japanese longsword, is drawn by grasping the saya near the top and pressing the tsuba with the thumb to emerge the blade just enough to unwedge the habaki from inside the saya in a process called “koiguchi-no-kirikata”. The blade is then free in the saya, and can be drawn out very quickly. This is known as “Koiguchi-o kiru”, nukitsuke, or “tanka o kiru” (啖呵を切る, “clearing the tanka”). This is obviously an extremely aggressive gesture, since a fatal cut can be given in a fraction of a second thereafter (see iaidō). It is similar in connotation and effect as drawing back the hammer of a handgun, chambering a round on a pump-action shotgun, or pulling back and releasing the charging handle on other firearms.
The expression “tanka o kiru” is now widely used in Japan, in the sense of “getting ready to begin something”, or “getting ready to speak”, especially with an aggressive connotation.
The habaki will cause normal wear and tear on the koiguchi and either a shim or new saya may be needed to remedy the issue as it will become too loose over time. Oiling under the habaki after cutting or once every few months is recommended by removing the habaki from the sword, though.