I. pə(r)ˈnishəs adjective
Etymology: Middle French pernicieus, from Latin perniciosus, from pernicies ruin, destruction (from per through + -nicies, from nec-, nex violent death) + -osus -ous — more at for , noxious
1. : highly injurious or destructive : tending to a fatal issue : deadly
a pernicious influence
pernicious habits
pernicious nonsense
a pernicious practice
pernicious disease
emphasis on fixed order frequently results in pernicious restraints against growing and vital movement — M.R.Cohen
no excuse for allowing our children to be taught a morality which we ourselves believe to be pernicious — Bertrand Russell
2. archaic : intending or doing evil : wicked , villainous
two pernicious daughters — Shakespeare
Synonyms:
baneful , noxious , deleterious , detrimental : pernicious describes that which harms exceedingly or irreparably by evil or insidious corrupting or enervating
pernicious social institutions which stifle the nobler impulses and encourage the baser — V.L.Parrington
addiction, on the other hand, carries with it a certain stigma which is not unjustified; it suggests the connotation of a pernicious or harmful repetitive act which gets out of the control of the individual — D.W.Maurer & V.H.Vogel
baneful may describe anything malevolent or malignant that is likely to kill, poison, or destroy
the baneful influence of this narrow construction on all the operations of the government — John Marshall
seen to be the outward projections of baneful subconscious elements and add up to a fearful indictment of the man — Times Literary Supplement
noxious may refer to what is at once harmful and unwholesome, corrupting, or noisome
the primitive plumbing of the 1870s, by conveying noxious odors into the rooms, was often a threat to the family health — A.M.Schlesinger b.1888
when the educator shall have been educated, the air cleared of noxious fallacies, and a sound and virile conception of learning restored — C.H.Grandgent
deleterious describes whatever has a harmful effect, often in some concealed or unguessed way
it was obvious that lime juice adulterated with five percent sulphuric acid, jellies with formaldehyde, peas with copper, cheap flavoring extracts with wood alcohol, and coloring matter with arsenic or mercury were highly deleterious to health — V.G.Heiser
heroin and other deleterious drugs — H.L.Ickes
detrimental is a general adjective for anything that harms
neutralizing or eliminating those influences in military aviation which are detrimental to the efficiency, health, or life of flying personnel — H.G.Armstrong
the wheat is cleaned and scrubbed and the fine hairs, detrimental to color and quality, removed — American Guide Series: Minnesota
II. adjective
Etymology: Latin pernic-, pernix swift (from perna haunch, ham) + English -ious — more at pearl
archaic : quick , swift