transcription, транскрипция: [ ɪnsɪdiəs ]
Something that is insidious is unpleasant or dangerous and develops gradually without being noticed.
The changes are insidious, and will not produce a noticeable effect for 15 to 20 years...
ADJ
• in‧sidi‧ous‧ly
Delusions are sometimes insidiously destructive.
ADV : usu ADV adj