men's changing room; men's powder room
Typically refers to a men's changing room in a gym, fitness club, or similar facility. The term パウダールーム is often associated with women's restrooms, so 男性用 clarifies it is for men.
I changed clothes in the men's changing room at the gym.
Compound of 男性用 (for men) and パウダールーム (powder room), a loanword from English. The exact origin is uncertain, but it likely arose in fitness club contexts to describe a men's changing area with amenities.