means A state of having no energy, enthusiasm, or inclination to do anything — a flat, drifting kind of tiredness of the spirit.

from From the old word 'list,' meaning desire or inclination — the same urge that makes you 'list' toward something you want. To be 'listless' is to have lost that pull entirely, and '-ness' simply bottles the condition. This 'list' is a cousin of the verb in 'do what you list' (do as you please) and is distantly related to 'lust,' both growing from a Germanic root for longing. The nautical 'list' — a ship leaning to one side — is a different word entirely, though there's a quiet poetry in how the listless person seems to lean, untethered, away from everything.