Impervious means: unaffected by, untouched by, immune to, invulnerable to, insusceptible to, resistant to, indifferent to, heedless of, insensible to, “he seemed impervious to the chill wind”, impervious to insults, impervious to fear
Of course no one is impervious to anything really, or they would be dead… You need to be present and notice attacks, malicious intention, fear, pain, and everything that you need to protect yourself from.
The impervious we are talking about is the emotional unshakeability that spares you from a life of a roller coaster, from being reactive, from being judgmental, for going into despair or going to mania.
The kind of inner environment where you can actually have joy, contentment, and workability. Where you “horse” doesn’t run away with you, hijacking you at the slightest irritation.