Longman Dictionary of Contemporary English homepage

Date: 1500-1600
Language: Latin
Origin: insidiosus, from insidiae 'attack from a hiding-place', from insidere 'to sit on, lie in wait', from sedere 'to sit'

insidious

 adjective
     
in‧sid‧i‧ous formal
an insidious change or problem spreads gradually without being noticed, and causes serious harm:
an insidious trend towards censorship of the press
—insidiously adverb
—insidiousness noun [uncountable]
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.
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