The definition of insidious is something or someone who works in a subtle or sly way, or in an intent to trap.(adjective)
An example of insidious is a plan that is meant to scam people out of money.
See insidious in Webster's New World College Dictionary
Origin: L insidiosus < insidiae, an ambush, plot < insidere, to sit in or on, lie in wait for < in-, in + sedere, to sit
See insidious in American Heritage Dictionary 4
Origin: From Latin īnsidiōsus
Origin: , from īnsidiae, ambush
Origin: , from īnsidēre, to sit upon, lie in wait for
Origin: : in-, in, on; see in-2
Origin: + sedēre, to sit; see sed- in Indo-European roots.
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