Inanition is what you feel when you're absolutely wiped out, utterly exhausted. You might be so weak that your inanition keeps you on the couch instead of meeting your friends for dinner.
Inanition is the total absence of energy and liveliness. Many people experience inanition after a difficult work week or a physically difficult task, and inanition is also common when you're sick or depressed. Another cause of inanition is not having enough to eat. The Latin root, inanitionem, actually means "emptiness," and comes from inanis, "empty, void, worthless, or useless."