core means core, from the French "core" which means circle. This word is also used to describe the central or most important part of something or matter.
Heartwood has several meanings, in biology, it is the innermost and hardest part of the trunk of a tree. It is the name of the inner layers of a tree that appear when cut across. It is the number of cores that identify the age of a tree.
In Brazil, heartwood is also the name of the part of the wood that remains intact after being burned. It is also the name given to the part of the wood that is out of the water, from a submerged trunk.
Core is also used to designate the essential part, be it a specific topic, a person, the inside of an individual. Core is the core, the heart, the most important piece of something or someone. For this meaning, the expression "heart of the matter". Ex: Let's not discuss other things right now, because that's the heart of the matter.
Core is still, in the figurative sense, the individual who, despite being old, is strong and robust.