INSIDIOUSLY

insidiously, perniciously

(adverb) in a harmfully insidious manner; “these drugs act insidiously”

Source: WordNet® 3.1

Etymology

Adverb

insidiously (comparative more insidiously, superlative most insidiously)

in an insidious manner

Source: Wiktionary

INSIDIOUS

In*sid"i*ous, a. Etym: [L. insidiosus, fr. insidiae an ambush, fr. insidere to sit in; pref. in- + sedere to sit: cf. F. insidieux. See Sit.]

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

2. Intended to entrap; characterized by treachery and deceit; as, insidious arts. The insidious whisper of the bad angel. Hawthorne. Insidious disease (Med.), a disease existing, without marked symptoms, but ready to become active upon some slight occasion; a disease not appearing to be as bad as it really is.

Syn.

– Crafty; wily; artful; sly; designing; guileful; circumventive; treacherous; deceitful; deceptive.

– In*sid"i*ous*ly, adv.

– In*sid"i*ous*ness, n.

Source: Webster’s Unabridged Dictionary 1913 Edition