curtain
幔 centers on a hanging cloth used to cover or divide: a curtain.
幔 combines 巾 (cloth) with 曼, which likely contributes the sound and the idea of something spread or extended, as a curtain spreads across an opening.
The cloth radical 巾 beside 曼 (which visually suggests something spread out) makes a curtain: a cloth that spreads across a window or doorway.
For マン, imagine a man pulling open a curtain: man -> マン, and the cloth 巾 spreads wide.