Etymology of Sidious 

insidious // adj. 
1 proceeding or progressing inconspicuously but harmfully (an insidious disease).
2 treacherous; crafty.
insidiously adv.
insidiousness n.

[Latin insidiosus ‘cunning’ from insidiae ‘ambush’ (as in-2, sedere ‘sit’)]

Perhaps it should be mentioned that the in- morpheme that prefixes the word insidious, is peripheral in meaningfulness, and that the "sidious" element has its root in LAtin insidiae ‘ambush’. So the word ambush should have some relevance in an etymological discussion. - CaptainSurrey