To be listless is to be lethargic, low spirited, and limp. If a fever has made you feel listless, you might also feel like you are melting into the sofa.
If you went to the supermarket without your shopping list, that doesn't mean you're listless. The word originates from the Middle English word liste which meant desire (and is related to our word lust). Oddly enough, listen comes from the same origin. If you're too lethargic and out of it to listen, you're listless. It's not just laziness, it's not just fatigue; an utter indifference to whatever is going on around you makes you listless.