insidious

As an adjective insidious

is producing harm in a stealthy, often gradual, manner.

As a noun insider is

a person who has special knowledge about the inner workings of a group, organization, or institution.

Canny is a related term of insidious.

As adjectives the difference between canny and insidious

is that canny is careful, prudent, cautious while insidious is producing harm in a stealthy, often gradual, manner.

As an adjective insidious

is producing harm in a stealthy, often gradual, manner.

As an adverb clandestinely is

in a secret or covert manner.

As adjectives the difference between shrewd and insidious

is that shrewd is showing clever resourcefulness in practical matters while insidious is producing harm in a stealthy, often gradual, manner.

Insidious is a synonym of concealed.

As an adjective insidious

is producing harm in a stealthy, often gradual, manner.

As a verb concealed is

(conceal).

Insidious is a related term of intriguing.

As adjectives the difference between insidious and intriguing

is that insidious is producing harm in a stealthy, often gradual, manner while intriguing is causing a desire to know more; mysterious.

As a verb intriguing is

.

As a noun intriguing is

an intrigue.

As adjectives the difference between insidious and sinister

is that insidious is producing harm in a stealthy, often gradual, manner while sinister is inauspicious, ominous, unlucky, illegitimate (as in bar sinister).

As adjectives the difference between spurious and insidious

is that spurious is false, not authentic, not genuine while insidious is producing harm in a stealthy, often gradual, manner.

As adjectives the difference between specious and insidious

is that specious is seemingly well-reasoned, plausible or true, but actually fallacious while insidious is producing harm in a stealthy, often gradual, manner.