Large heavy metal spike.
Essentially an oversized nail, spikes made out of metal are used to fasten large objects that are too big for small nails. Metal spikes are most commonly used to fasten railroad tracks into the ground, which must be strong and durable enough to withstand the weight and tremor of passing trains. Particularly crafty tinkerers may also find a more violent use for metal spikes, and with the arrival of the zombie apocalypse, this became its main use.
Can be found in hardware stores and warehouses, or illegally pulled from railroad tracks.