Sluggish, a.
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 sluggish; he lists it under idle, slow.
Where he writes about it:
The inert person seems like dead matter (characterized by inertia), powerless to move; the sluggish moves heavily and toilsomely; the most active person may sometimes find the bodily or mental powers sluggish. — from idle
Slack refers to action that seems to indicate a lack of tension, as of muscle or of will, sluggish to action that seems as if reluctant to advance. — from slow