Since it is tapered, I think it is something used to spread something or as a splitting wedge. I think railroad spikes and others used to hold things together are pointed, but have consistent size for most of the spike. This spike was probably driven into the end of a tenon after insertion into a mortise to tighten the joint, or as a simple splitting wedge for firewood. The reason I have doubts about using it as a splitter is because the head is flared. Most splitting wedges I have seen have no head on them.