Adjective

1 a.not allowing entrance or passage; impenetrable

b. not capable of being damaged or harmed

2not capable of being affected or disturbed

http://www.merriam-webster.com/word-of-the-day/

I ‘ve learned this word from Merriam-Webster, Word of the day, on Dec 07. At the same time, I’ve also learn several prefix, like im- which means opposite, per- means though, via means way.