The phrase "insidious nature" refers to something that is sneaky, gradual, and harmful. It often describes something that slowly causes damage or harm while remaining unnoticed or difficult to detect. Full definition
The western genre, long associated either with American film and the lone cowboy holding fast to independence in a changing landscape, or the spaghetti western, in which European directors examined more forcefully the often insidious nature of the more corrupt... (screenanarchy.com)
The western genre, long associated either with American film and the lone cowboy holding fast to independence in a changing landscape, or the spaghetti western, in which European directors examined more forcefully the often insidious nature of the more corrupt side of the cowboy / frontier life, is given fresh eyes in Five Fingers for Marseilles. (screenanarchy.com)
And therein lies the real insidious nature of the happiness movement that's sweeping our culture. (entrepreneur.com)