`  == 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`