noun
marking pin; tailor's pin
A pin used in sewing to temporarily hold fabric or mark positions before stitching.
布を切る前に、まち針で印をつける。
Before cutting the fabric, I mark it with marking pins.
まち針を一本落としてしまったので、気をつけてください。
I dropped a marking pin, so please be careful.
Compound of 待ち (machi, 'waiting') and 針 (hari, 'needle'), referring to a pin that 'waits' before final stitching. The spelling まち針 is common.