insidious

 
Pronunciation: /ɪnˈsɪdɪəs/

adjective

  • proceeding in a gradual, subtle way, but with very harmful effects: sexual harassment is a serious and insidious problem

Derivatives

insidiously

adverb

insidiousness

noun

Origin:

mid 16th century: from Latin insidiosus 'cunning', from insidiae 'an ambush or trick', from insidere 'lie in wait for', from in- 'on' + sedere 'sit'