An Etymological Dictionary of Astronomy and Astrophysics

Having no inherent power of action, motion, or resistance (opposed to active).
Chemistry: A chemically inactive element, compound or material. → inert gas.

Etymology (EN): Inert, from Fr. inerte, from L. inertem “unskilled, inactive,” from → in- “without” + ars (gen. artis) “skill.”

Etymology (PE): Laxt “sluggish, inert.”