1. 隐伏的;潜在的;不知不觉间加剧的
Something that is insidious is unpleasant or dangerous and develops gradually without being noticed.

1. working or spreading in a hidden and usually injurious way

e.g. glaucoma is an insidious disease
a subtle poison

Synonym: pernicioussubtle

2. intended to entrap

3. beguiling but harmful

e.g. insidious pleasures