noun
A device placed in or over the mouth to prevent speech or noise. Often used in BDSM contexts, hence the vulgar label. Compare 猿轡 (さるぐつわ), a similar gag.
See also: 猿轡
彼女は口枷をはめられて声を出せなかった。
She was gagged and couldn't make a sound.
猿轡 is a more specific term for a gag that is tied around the head, often a cloth or rope, while 口枷 can refer to any mouth gag, including ball gags.
Compound of 口 (くち, 'mouth') and 枷 (かせ, 'shackles, fetters'). The kanji 枷 is not commonly used in modern Japanese outside of this word and a few others.