Why do we say that we know things by heart?
When I was a kid, rather than know things ‘by heart’, we’d know things ‘off by heart’…
As to the OP; no idea…
Not a scholarly dissertation here, so if anyone catches me in a supposition or outright error, feel free to correct me.
I do believe that the ancients considered the heart to be the seat of the soul. Or if they didn’t believe the actual organ housed the soul, then their term for what comprised our essence was “heart.”
Perhaps Zev, Chaim, Dex or another of those more familiar with Scripture can shed more light, but I know of at least one Scripture verse which alludes to this (“Thy word have I hid in my heart that I might not sin against thee.” Psalm 119:11) My concordance lists many cites for “heart” and it would be naive to think that this was the only verse that uses the word this way.
Thus, knowing something “by heart” would mean that it has become a part of our very essence and being.
Similarly, I took the meaning to be “know something in the core of one’s being, without thought or analysis or remembering”. In a like manner, to feel something “in my heart” would be to feel it deeply and without complicating distractions. I expect a linguistic reference to the heart to imply simple conviction, in contrast to detailed and complex processes of the intellect.