tenuis

Tenuis, tenue is third declension with two terminations: masculine and feminine share tenuis, the neuter is tenue, ablative singular tenui, genitive plural tenuium. It sits with gravis, levis and brevis in that pattern.

The root is the one in tendo, "stretch": what has been stretched out is thin. From there the word runs across three registers - the physical (thin thread, rarefied air, shallow water), the social (tenuis of a person means of small means, and the tenuiores are a legal class), and the stylistic (the plain, unadorned manner of writing). The verb is tenuo, "to make thin", with attenuo beside it. English got tenuous, tenuity and attenuate; French ténu.