thinner futon placed against the skin
A lightweight futon used directly against the body, often under a heavier futon or blanket. Common in traditional Japanese bedding.
On cold nights, I layer a thick quilt over the skin-side futon.
敷き布団 is the mattress-like futon laid on the floor, whereas 肌布団 is a thin layer used for warmth and comfort against the body.
Compound of 肌 (hada, 'skin') and 布団 (futon, 'futon, bedding'). The word describes a futon that touches the skin directly.