noun
A short pin with a flat, broad head, used to attach paper or light items to a bulletin board or wall.
ポスターを画鋲で壁に留めた。
I pinned the poster to the wall with a thumbtack.
画鋲を踏んでしまって痛かった。
I stepped on a thumbtack and it hurt.
画 (picture) + 鋲 (rivet, tack). The kanji 鋲 is a kokuji (Japanese-created character) combining 金 (metal) and 兵 (soldier/weapon), suggesting a metal fastener.