means Lacking energy, enthusiasm, or interest, drifting through tasks without any drive to do them.

from From Middle English "listles" — "list" here is the old noun meaning desire or inclination (from Old English "lystan," to please or wish for, a cousin of "lust"), plus the suffix "-less." So to be listless is, quite literally, to be without wanting. The same "list" survives in the archaic verb "as you list" (as you please), now nearly extinct, leaving "listless" as its rather glum surviving relative.