I agree with Chris (I've heard 'inimical' but had to look it up). But if you have to know them . . . . INIMICAL . . . . part of it phonetically sounds like ENEMY and it means hostile, etc. INIMITABLE . . . . is actually pretty straightforward: IMITABLE = Able to imitate. IN prefix = Not, so the entire word = Not able to be imitated.

I can tell you that, as a native speaker I don't know the first word, and while I do know the second one, it's not a word I would be likely to use. They are both very technical words, and I try not to use technical words. :)