We're reading “Kokoro” （『こころ』) by Natsume Souseki in my Japanese Literature class. During the lecture our teacher told us a bit about the etymology of the word kokoro, heart.
First, there's an onomatopoeic origin. Koro koro (ころころ) is the sound that something makes, for example, when it's rolling quickly across the floor. Its sound is also reminiscent of the "lub-dub" of your heart's beat, don't you think? It can also mean something that is easily overturned, or set into motion: for example, a conversation can suddenly, ころころと, turn to darker themes. I thought it was interesting that the sound, perhaps, of a heart could be related to the idea that it is something that is easily moved, or that ころころと changes.
The second word that our professor mentioned is the word kogoru (こごる、凝る), which means hard, frozen, or to coagulate. This seems to refer to the fact that the blood in your body hardens when your heart is no longer koro koro-ing. こごる can also signify a state in which you cannot break yourself free (ie, your arms and legs are bound). This brings to mind the way that your heart can be 'frozen' or 'captivated' by one thing. In fact that same kanji shows up in the word koru"(凝る), to be fanatic.
Thinking about the English "heart"... We use it in so many different phrases to signify so many different things. The center of something is the heart; you can believe something by 'taking it to heart'; someone with similar preferences to me is someone "after my own heart"; you can remember someone by keeping them "in your heart"; your emotions and opinions may be revealed if you "wear your heart on your sleeve"; a person who is too sympathetic is a "bleeding heart"; and so on. But I have trouble pondering the etymology of the word. It seems to stand on its own in my head. It isn't like the word inspiration, that you can break into Latin...