spike; very large nail
Civil engineering term for a large nail or spike, used in heavy construction such as timber framing, railway ties, or bridge building.
橋の工事で、大釘を使って木材を固定した。
During the bridge construction, they used spikes to secure the timber.
Compound of 大 (おお, 'large') and 釘 (くぎ, 'nail'). The reading おおくぎ follows standard rendaku rules.