imperturbable

adjective1

1. •
   Not capable of being, or not easily, perturbed, excited, or upset; calm and collected, even under pressure.

   • [T]o a vvise man there can happen no iniury or offence at all, to molest the felicity of his minde, vvhich (in the Stoicks opinion) ought to bee imperturbable, and his heart adamantine. — 1605