Refers to a U-shaped metal fastener driven into wood or other materials, similar to a staple used with a staple gun.
See also: ステープル
I used a staple to bind the documents together.
I fastened the poster to the wall with a staple.
Standard kanji-kana mixed spelling for 'staple' (fastener).
Alternate kanji spelling using 股 (crotch/fork), but また釘 is more common.
Compound of また (fork/crotch) and 釘 (nail), describing the U-shaped fastener that resembles a forked nail.