Pernicious means very harmful or evil, and often in a quiet way, so you do not notice the damage at first. A pernicious rumour can slowly ruin someone’s name. A pernicious habit can eat at your health for years before you see it. The word is formal and a little literary; you will hear it in essays, news, and medicine more than in the playground. In medicine, pernicious anaemia is a real illness name — a serious kind of anaemia — and that set phrase is worth knowing as a block. Though actually, it does not mean “precious,” and it does not simply mean “annoying.” The harm is real and often deep.