It's the girl in a small size, Brandi Melville (bm for short). This is a clothing brand that only sells small clothes. If you are a little fatter, you can't put it on at all. Pants are 25 inches (64 cm) around the waist, and only very thin girls can wear them. People sometimes try to lose weight by wearing bm and become BM girls.

The source of this stalk:

Brandy Melville (BM), which is the abbreviation of Brandy Meville, is an Italian brand. BM only provides clothes in small sizes, so if you want to wear them, you must be slim enough. Many girls use this to prove their figure and become BM girls.

The usage of this stalk:

The weight is obviously thin, and she is a size S girl, which can be called BM girl.

This BM girl is so beautiful.