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