Insidious

[PussyCat]

Lying in wait; watching an opportunity to insnare or
entrap; deceitful; sly; treacherous; -- said of persons; as, the
insidious foe.

[Hina Muqadas]

Proceeding in a gradual, subtle way, but with very harmful effects.

[Sommer]

[Insidious] (adverb insidiously)

developing in a very gradual, in a [stealthy] manner, with the [intend] of causing damage.

[Sommer]

A [scary] [film]

[Luis Fok]

[The Devils] [coochie]/vagina. Can also be used as an adjective to describe the temperature of the surrounding [climate].

[Julene]

(of something unpleasant or dangerous) gradually and secretly causing harm: