1. proceeding in a gradual, subtle way, but with harmful effects: "sexually transmitted diseases can be insidious and sometimes without symptoms"
▪ treacherous; crafty:"tangible proof of an insidious alliance"
Word Originmid 16th century: from Latin insidiosus ‘cunning’, from insidiae ‘an ambush or trick’, from insidere ‘lie in wait for’, from in- ‘on’ + sedere ‘sit’.