noun
double-pointed nail; dowel
A nail or pin with points at both ends, used to join two pieces of wood together; also refers to a dowel used in joinery.
合い釘で二枚の板をしっかりと接合した。
I joined the two boards firmly with a double-pointed nail.
この家具は合い釘を使って組み立てられている。
This piece of furniture is assembled using dowels.
ダボ is a more common modern term for a wooden dowel, while 合い釘 can refer to both metal double-pointed nails and wooden dowels.
Compound of 合い (ai, 'joining') + 釘 (kugi, 'nail'). The spelling 間釘 uses 間 (ai, 'space, interval') for the same reading.