Osage Orange.
Most everyone's nailed it. We did some TSI (Timber Stand Improvement) on the property about 6 years ago. We have a fair amount of Osage Orange. They can die and still be standing there 50 years or longer with no noticeable change!
The previous owner had a few fence row posts made out of it.
I never try to cut it for firewood until its been dead a few years. Even then, I bring a freshly sharpened saw and a few extra chains.
I always try to mix it with other hardwoods when burning like others have mentioned.
Earlier poster nailed it. I saw a firewood burn chart that had it right below coal for heat production as well as being one of the most dense.
As soon as I open my wood burning stove's door, look out. When the O2 hits it it sparks everywhere.
The "hedge apples" that drop make great targets!