insidious

As an adjective insidious

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

As a noun incubation is

sitting on eggs for the purpose of hatching young; a brooding on, or keeping warm, to develop the life within, by any process.

As adjectives the difference between insidious and insipient

is that insidious is producing harm in a stealthy, often gradual, manner while insipient is foolish; lacking wisdom; stupid.

Treasonable is a synonym of insidious.

As adjectives the difference between treasonable and insidious

is that treasonable is involving or constituting treason while insidious is producing harm in a stealthy, often gradual, manner.

Fraudful is a related term of insidious.

As adjectives the difference between fraudful and insidious

is that fraudful is full of fraud, deceit, or treachery; fraudulent while insidious is producing harm in a stealthy, often gradual, manner.

As adjectives the difference between indolent and insidious

is that indolent is habitually lazy, procrastinating, or resistant to physical labor/labour while insidious is producing harm in a stealthy, often gradual, manner.

As adjectives the difference between insidious and sullen

is that insidious is producing harm in a stealthy, often gradual, manner while sullen is having a brooding ill temper; sulky.

As a noun sullen is

(obsolete) one who is solitary, or lives alone; a hermit.

As adjectives the difference between insidious and seductive

is that insidious is producing harm in a stealthy, often gradual, manner while seductive is attractive, alluring, tempting.

As adjectives the difference between insidious and cumulative

is that insidious is producing harm in a stealthy, often gradual, manner while cumulative is incorporating all data up to the present.

Private is a related term of insidious.

As adjectives the difference between private and insidious

is that private is belonging to, concerning, or accessible only to an individual person or a specific group while insidious is producing harm in a stealthy, often gradual, manner.

As a noun private

is the lowest rank of the army.

Traitorous is a related term of insidious.

As adjectives the difference between traitorous and insidious

is that traitorous is characteristic of a traitor; disloyal while insidious is producing harm in a stealthy, often gradual, manner.