The term, insidious, has been recently made famous by the popular movie, Insidious, which released in 2010. The horror movie was so popular that it spawned a sequel, Insidious 2, which released in 2013, and Insidious 3, which is set to release on June 5, 2015.
The term, insidious, refers to something that proceeds in a slow and subtle way, but with harmful effects. It is intended to entrap and can be treacherous. It refers to an object that is alluring and mesmerizing, but is actually seeking to do harm. The insidious object is subtle, stealthy, cunning, and treacherous. It works in slow and subtle manner, and lies in wait to entrap its victims.
The English ‘insidious’, is derived from the Middle French ‘insidieux’, which is in turn take from the Latin ‘insidiosus’, meaning “cunning, artful, deceitful”. The Latin term comes from another Latin word, ‘insidiae’ which means “lying in wait, an ambush, artifice, or stratagem”.
So, basically insidious means something that is harmful but attractive, something that waits in turn to cause harm, or something that moves ahead in a slow and subtle way, while slowly causing harm and damage. For example: an insidious disease.