corners of the mouth; lip corners; commissures
Refers to the outer corners where the upper and lower lips meet. The term is neutral and used in both everyday and medical contexts.
笑うと口尻が上がる。
The corners of my mouth lift when I smile.
口尻が切れて痛い。
The corners of my mouth are cracked and painful.
Compound of 口 (mouth) and 尻 (buttocks, end), literally 'mouth end', referring to the corners of the mouth. The exact historical derivation is uncertain, but the kanji combination is transparent.