insidious

Từ điển Oxford Advanced Learner 8th

in·sidi·ous BrE [ɪnˈsɪdiəs] NAmE [ɪnˈsɪdiəs] adjective (formal, disapproving)
spreading gradually or without being noticed, but causing serious harm
•the insidious effects of polluted water supplies
Derived Word: ↑insidiously

Word Origin:
[insidious insidiously] mid 16th cent.: from Latin insidiosus ‘cunning’, from insidiae ‘an ambush or trick’, from insidere ‘lie in wait for’, from in- ‘on’ + sedere ‘sit’.

• Từ đồng nghĩa / Synonyms:
  pernicious, subtle