Definition "C" comes from, it's really hard to say, some proponents of the idea say that it's very old, although I don't think there isn't any historical evidence to prove such a claim.
Have you tried to compare texts of Ueshiba to texts from qi gong?
There is a chapter in "The secret teachings of Aikido" that is named "Aiki is the Marvelous Functioning of Breath". If you have a comprehensive book on qi gong, you may read the capter of Ueshiba and try to use the qi gong book as a "dicitionary". I don't know, whether this works, when one does not practice qi gong or tai chi or another comparable art. But if so you will be able to translate Ueshibas words. Not only it's meaning, but you often will be able to translate them right into certain movements.
For different reasons this don't will work in an exact way. There will be many questions, depending on the translation of Stevens, on the different schools and ways of qi gong. And so on ...
But: You will get some clue, what Ueshiba is talking about.
(And he is clearly talking about something that is goining on within your own body. The relationship to a different person or body or society is something that can and will happen when the individual realizes aiki. You can use aiki in interaction. But the interaction is not described as "being" aiki in which way ever.)
This is about what the term "aiki" means. It's not about where the term itself comes from, is used first time and so on. This is a different matter.
So I think, aiki is not "a body ability", but it is a way to understand the interplay of body, qi and mind. To form and to train this interplay by certain methods. And finally - if possible - to use this interplay. Be it for getting healthier. Or be it martially.