small opening under the sleeve of traditional Japanese clothing
Refers to the slit or opening where the sleeve meets the bodice, below the armpit, found in garments like kimono or yukata. A specialized term used mainly in the context of traditional clothing.
The breeze comes in through the opening under the sleeve of the kimono, making it cool.
The stitching of the underarm opening is coming undone a little.
The term 八つ口 (やつくち) literally means 'eight mouths' or 'eight openings', but the exact origin of the name is uncertain. It may refer to the shape or the number of openings in traditional clothing, though this is speculative.