Synonyms of the word

pernicious

• adj. Causing much harm in a subtle way.
• adj. Causing death or injury; deadly.

baneful

• adj. Exceedingly harmful; causing harm, death, ruin.
• adj. Deadly or sinister.
• adj. Portending disaster.
• adj. (archaic) Poisonous.

deadly

• adj. (obsolete) Subject to death; mortal.
• adj. Causing death; lethal.
• adj. Aiming or willing to destroy; implacable; desperately hostile.
• adj. (by extension) Very accurate (of aiming with a bow, firearm, etc.).
• adj. (informal) Very boring.
• adj. (informal) Excellent, awesome, cool.
• adv. (obsolete) Fatally, mortally.
• adv. In a way which suggests death.
• adv. Extremely.

harmful

• adj. of a kind likely to be damaging; injurious.

insidious

• adj. Producing harm in a stealthy, often gradual, manner.
• adj. Intending to entrap; alluring but harmful.
• adj. (nonstandard) Treacherous.

noxious

• adj. Harmful; injurious.

pestilent

• adj. Highly injurious or destructive to life: deadly.
• adj. (informal) Annoying.
• adj. (archaic) Harmful to morals or public order.

subtle

• adj. Hard to grasp; not obvious or easily understood; barely noticeable.
• adj. (of a thing) Cleverly contrived.
• adj. (of a person or animal) Cunning, skillful.
• adj. Insidious.
• adj. Tenuous; rarefied; of low density or thin consistency.