Since ki is used in the Japanese language as part of hundreds of idioms ( see Jun's list below ) and O'Sensei undoubtedly knew this, maybe the ki in Aikido is meant to be an open ended idiom, no single definition but many.
In essence, the character ki means:
* spirit, mind, soul, heart
* bent, interest
* mood, feeling
* temper, disposition, nature
* care, attention
* air, atmosphere
* energy, essence, air, indications
* touch, dash, shade, trace
* spark, flash
Here are some contexts in which the word "ki" and some of its derivations are used in everyday Japanese.
Japanese Phrase Kanji Literal Translation Definition
Gen ki "source/foundation of ki." one's health
Byou ki "ill ki." to be sick
Ten ki "heavenly ki." the weather
Ki ga tatsu "the ki stands upright." to get angry
Ki wo tsukeru "to put on (or to have) ki" to be careful; to be attentive
Ki ga kiku "the ki is used a lot" to be empathetic
Ki ga susumanai "the ki does not go forward." to not want to do something
Ki ga sumu "the ki is finished or used up." to feel fulfilled
Ki ga tsuku "to have "ki" put onto you." to notice
Ki ga tsuyoi "the ki is strong." to be headstrong
Ki ga yowai "the ki is weak." to be like a coward
Ki ga tooku naru "the ki goes far away." to become lightheaded
Ki ga nai "to have no ki" to have no interest in something
Ki ga nukeru "the ki becomes missing." to lose hope
Ki ga mijikai "the ki is short." to be short tempered
Ki ni sawaru "something touches the ki." to find something irritating
Ki ni naru "to become ki" to have something nagging or on one's mind
Ki wo kubaru "to pass out ki (to people)" to attend to other people's wishes"
And I would add,O'sensei wanted us to practice in an atmosphere of joy then your Aikido could be funki.