Impassive a. : Not susceptible of pain or suffering; apathetic; impassible; unmoved.

Example:
So what exactly is the difference between the words passive and Impassive?

Taxonomy
emotion, Merriam-Webster Online Dictionary, sive, Dictionary, webster, siv, Word Games, pain, Webster Online Dictionary with audio pronunciations, absence, thesaurus, stoic, phlegmatic, glossary, english, language, ESL, Search.

Defination from the web:
impassive from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... synonyms impassive, stoic, phlegmatic, apathetic, stolid mean unresponsive to something that might normally excite interest or emotion. ...