noun
obi padding; sash stiffening; thick obi sash
Refers to the inner padding or stiffening material sewn inside an obi (kimono sash) to give it body and shape. Can also refer to a thick obi itself, though the padding sense is more common.
帯芯を入れると、帯に張りが出て結びやすくなる。
Inserting obi padding gives the sash firmness and makes it easier to tie.
この帯は帯芯が厚くて、しっかりとした仕上がりだ。
This obi has thick padding and a sturdy finish.
Compound of 帯 (obi, 'sash') and 芯 (shin, 'core, wick, padding'). The spelling 帯心 is a variant using 心 (heart/core), but 帯芯 is the standard form.