adjective

insidious

in-SID-ee-uhs/ɪnˈsɪdiəs/

Causing harm gradually in a way that is hard to notice.

The termites' insidious damage hollows the post while its outer surface appears sound.

An apparently sound wooden support post conceals extensive termite damage: an illustrative cutaway reveals branching galleries hollowing out the interior beneath a thin, intact surface.

Etymology

inLatinin, on

+sidLatinto sit

+iousLatinfull of, characterized by

“Sitting in or on” led to the image of an ambush: lying in wait to entrap. Insidious harm similarly works deceptively, developing without making its danger obvious.

In context

Dr Ashok Jansari NeuroTalk65:16

dementia is extremely prevalent across the world but it's usually diagnosed when it's too late because it's a subtle insidious disorder that comes on very slowly

Confabulation, Capgras & Cotard's: The Amazing Insights From Brain Disorders (part 1)

More examples

The leak causes insidious damage behind the kitchen walls.
Repeated small insults can have an insidious effect on confidence.
The disease is insidious, progressing for years before symptoms appear.

Forms

insidious

insidiously adverb — In a gradually harmful, hard-to-notice way.

insidiousness noun — The quality of causing gradual, unnoticed harm.

Similar words

pernicious: Harmful in effect, but not necessarily gradual or hidden.
corrosive: Gradually destructive, especially to trust, confidence, or relationships.