drawing by marguerita
Mathematically, chaos means an aperiodic deterministic behavior which is very sensitive to its initial conditions, i.e., infinitesimal perturbations of initial conditions for a chaotic dynamic system lead to large variations of the orbit in the phase space.
Chaotic systems are systems that look random but aren't. They are actually deterministic systems (predictable if you have enough information) governed by physical laws, that are very difficult to predict accurately (a commonly used example is weather forecasting).
Furthermore, the word gas is probably an alteration of chaos. Particles in gases exhibit chaotic motion, although this was unknown to Jan Baptist van Helmont, the inventor of the term. He is instead believed to have been influenced by the concept of chaos in the occult theories of Paracelsus.