Something that is insidious is unpleasant or dangerous and develops gradually without being noticed.
例句
The changes are insidious, and will not produce a noticeable effect for 15 to 20 years...变化是悄然发生的,15 到 20 年间不会产生明显影响。They focus on overt discrimination rather than insidious aspects of racism.他们关注的是公开的歧视,而不是种族主义的一些潜在问题。