You'll have more STHENIA if you eat your spinach

What's the word I'm thinking of? Today, it's...

STHENIA [STHEE-nee-uh] (noun)

strength, energy, vitality 

"The first condition he called sthenia," I explained. "Meaning lots of energy." "Correct. And the other group, by distinction had a-sthenia." "No energy." "Correct again. You do know your etymology, Samantha."
--Beach Conger, It's Probably Nothing: More Adventures of a Vermont Country Doctor (2011)