From Middle English, from Old French pernicios, from Latin perniciōsus, from perniciēs ("destruction"), from per ("through") + nex ("slaughter, death").

• (RP) IPA: /pəˈnɪʃəs/
• (America) IPA: /pɚˈnɪʃəs/

pernicious

1. Causing much harm in a subtle way.

   Synonyms: deleterious, venom

   • c. 1603–1604 (date written), William Shakespeare, “Measure for Measure”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act II, scene iv], page 70 ↗, column 1:

     Ang. Belieue me on mine Honor,
     My words expreſſe my purpoſe.
     Iſa. Ha? Little honor, to be much beleeu'd,
     And moſt pernitious purpoſe: Seeming, ſeeming.
     I will proclaime thee Angelo, looke for't.

   • 1838, [Letitia Elizabeth] Landon (indicated as editor), chapter XVIII, in Duty and Inclination: […], volume III, London: Henry Colburn, […], →OCLC ↗, page 231 ↗:

     […] her health also having suffered by the change, she immediately, when told of Douglas's projected departure from India, felt the strongest desire to accompany him; and to which Colonel Melbourne the more readily consented, having with reluctance yielded to her request of quitting England, willing as he had been to sacrifice the enjoyment of her society rather than that she should submit to the disadvantages attendant upon a residence in a clime usually found so pernicious to the female constitution.

2. Causing death or injury; deadly.

   Synonyms: attery

3. (of a person) Insidiously villainous: intending to cause harm, especially in a subtle way.

• French: nocif, malsain, pernicieux
• German: schädlich, bösartig, verderblich, (medicine) perniziös
• Italian: pernicioso
• Portuguese: pernicioso
• Russian: тлетво́рный
• Spanish: pernicioso

• French: nocif, délétère, nuisible, dangereux
• German: (medicine) perniziös, tödlich
• Portuguese: mórbido
• Russian: вре́дный
• Spanish: pernicioso

Analyzable as pernīx ("swift") + -ious.

pernicious

1. (obsolete) swift; celeritous.

• pernicity