noun
collar core; collar stiffener
Refers to the stiff fabric, interlining, or plastic strip inserted inside a shirt or kimono collar to keep its shape.
ワイシャツの衿芯が曲がってしまった。
The collar stay of my dress shirt got bent.
衿芯を入れ替えると、襟の形がきれいになる。
Replacing the collar stiffener makes the collar shape look neat.