English
insidiousness
From insidious + -ness.
A compound of insidious + -ness
Proto-Indo-European *sed- ‘to sit’
…
Latin īnsideō ‘to sit in or on’
Latin īnsidiae ‘a lying in wait, an ambush, artifice, stratagem’
Latin īnsidiōsus ‘cunning, artful, deceitful’
Middle French insidieux
English insidious
Today it means
noun A surreptitious harmfulness, quality of entrapment, or treacherousness; the characteristic of being insidious.
Language periods are conventional ranges, not dates for this word.
Etymograph / insidiousness