Tenuous, a.
Etymology: [L. tenuis thin. See Thin, and cf. Tenuis.]
1. Thin; slender; small; minute.
2. Rare; subtile; not dense; -- said of fluids.
Lacking substance, as a tenuous argument.
Shades of meaning
Fernald has no entry of its own for tenuous; he lists it under fine.
Where he writes about it:
As that which is delicate is apt to be small, by an easy extension of meaning fine becomes a synonym for slender, slight, minute, comminuted; as, a fine thread, fine sand; or for filmy, tenuous, thin; as, a fine lace, fine wire; and as a thin edge is keen, sharp, fine becomes also a synonym for these words; as, a fine point, a fine edge. — from fine