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.
The corners of my mouth lift when I smile.
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.