Sluggish, a. — Webster enters it under “sluggish”
1. Habitually idle and lazy; slothful; dull; inactive; as, a sluggish man.
2. Slow; having little motion; as, a sluggish stream.
3. Having no power to move one's self or itself; inert. Matter, being impotent, sluggish, and inactive, hath no power to stir or move itself. Woodward. And the sluggish land slumbers in utter neglect. Longfellow.
4. Characteristic of a sluggard; dull; stupid; tame; simple. [R.] "So sluggish a conceit." Milton.
1 more senses
Syn. -- Inert; idle; lazy; slothful; indolent; dronish; slow; dull; drowsy; inactive. See Inert. -- Slug"gish*ly, adv. -- Slug"gish*ness, n.
Shades of meaning
Fernald has no entry of its own for sluggishness; he lists it under apathy, stupidity.
Where he writes about it:
There are persons to whom a certain degree of apathy is natural, an innate sluggishness of the emotional nature. — from apathy
Stupidity is sometimes loosely used for temporary dulness or partial stupor, but chiefly for innate and chronic dulness and sluggishness of mental action, obtuseness of apprehension, etc. — from stupidity