"so did the world and inertia creep into Siddhartha's soul"

Inertia comes from the Latin word, iners, meaning idle, or lazy. Isaac Newton defined inertia as follows: “The vis insita, or innate force of matter, is a power of resisting by which everybody, as much as in it lies, endeavors to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.”

From a metaphysical perspective, soul inertia is when a person becomes complacent in his material existence, unable to accept change, or break the shackles of mundane matters. Inertia of the soul prevents a person to seek higher goals of self-realization and salvation.