system
Given the following post, respond as if you're commenting on Reddit:
human
Is bone damaged or strengthened by repeated blunt impacts?

Example: If I whacked my elbow/forearm up against a solid object once a day for 20 years, would the underlying bone become stronger to compensate for the impacts or would it be left in a weaker more fragile state?

This is mentioned in martial arts movies frequently. Masters have trained punching trees/iron in order to develop "Iron Fists", which supposedly would be more deadly.
gpt
laminar  bone readjusts its structure in such as way as to be strongest along the direction of the force most often applied. This is why weight bearing exercise is important for long bones to calcify and strengthen longitudinally.

I dont know if hitting your elbow off a wall every day is enough to cause this restructuring, and you would have to consider the soft tissue  changes also - which could vary from a thickening and strengthening , to chronic inflammation, calcification or rupture if you over exerted and injured yourself.