This word has Latin roots that mean “sitting in,” as in “sitting in a hiding place, lying in wait, about to trap or attack someone.”

Insidious people and things are secret, sneaky, and tricky, as if they’re planning to trap things or other people.

in SID ee us

Part of speech:
Adjective: “their insidious glances,” “that plan was insidious.”

Other forms:
insidiously, insidiousness

How to use it:
Use this formal, sharply critical word to talk about insidious people and their insidious plots, plans, and intentions.

Or, talk about insidious diseases; insidious issues and problems; insidious power, reach, and influence; insidious greed, corruption, and racism; insidious lies, myths, rumors, and misconceptions, etc.

Something might play an insidious role, reveal an insidious side or aspect, or have an insidious impact, effect, outcome, or implication.

And feel free to get abstract: to exaggerate and personify. “With a crafty widening of her sapphire eyes and an insidious tilt of her furry head, Layla lures me in. Work can wait; she needs petting now.”

“Especially insidious are electronic cigarettes, which are highly addictive and also lead teens toward traditional cigarette use.”
— Editorial Board, Seattle Times, 30 December 2018

“But chemicals in the environment often enter skin in more insidious ways… Many airborne pollutants are so small that they enter pores in the skin like pebbles dropping into a cup… Others… can easily pass through the fat-filled spaces between skin cells. They then enter the circulatory system, where they can cause widespread effects.”
— Elizabeth Svoboda, Nature, 21 November 2018