The word is only one of my favorites; many that earn the title are onomatopoeic in some way. insidious sounds like a snake, a reptile with a biblical and cultural reputation for treachery and perniciousness. (Perniciousness is the same: I sneer when I say it, my face befitting its meaning.)
I said it before Louis C.K. did, but he said it better: the word is overused, rendering it meaningless. How can someone's new haircut be amazing (maybe if it were on fire?)? How can a trip to Italy, a gelato in said country, and the warm washcloth provided by the airlines all be amazing? I avoid using the word, but when I was pregnant, I kept thinking, "Now THIS is amazing."