noun
Wasei-eigo from 'running shirt'. Refers to a sleeveless top worn as sportswear or as an undergarment. In Japanese, it is not limited to running contexts.
wasei-eigo (Japanese-coined English), from running shirt
夏はランニングシャツ一枚で過ごす。
In summer, I spend the day in just a tank top.
運
When I exercise, I change into a tank top.