Oominoo is truly the ultimate word. It means both everything and nothing. Oominoo can be used in any situation such as; "your shirt is so...so...oominoo! or "run, the teacher has gone oominoo!"
Also oominoo could be referring to the god Oominoo who the cult of oominoo believes starteed out as a lowly word but acheives enlightenment by just being so awsome.
When Mumbo turns you into a creature he says "Oominaka." Grant Kirkhope, Mumbo's voice actor, revealed that at the time of recording the voices for the game his left testicle was swollen. "Knackers" is British slang for testicles, so he thought it would be funny if Mumbo said "Ooh, my Knackers." Thus, every time that Mumbo uses his magic to turn you into a creature, he's actually complaining that his testicles are in pain.
"How are you feeling today?"
"Not good, I think my testicles are swollen"
*kicks him in the balls*
"Oominaka!"
"Oh, my testicles hurt!"
"Oh, my balls are really sore!"