An Etymological Dictionary of Astronomy and Astrophysics

Tendency of a body to preserve its state of rest or uniform motion in a straight line.

Etymology (EN): Inertia, from L. inertia “un-skillfulness, idleness,” from iners (gen. inertis) “unskilled, inactive;” → inert.

Etymology (PE): Laxti “sluggishness, inertia.”