What is Weird Words? Read the foreword here.

Pernicious
adjective
1. causing insidious harm or ruin; ruinous; injurious; hurtful
2. deadly; fatal.
3. Obsolete evil; wicked.

The sound of the word pernicious (at least to me) sounds pleasant, a striking contrast to its definition. Something pernicious is intended to harm, or perhaps be fatal. A pernicious virus is one that will cause considerable damage to the carrier, if not killing them. Defamatory remarks could also be considered pernicious if their intent is to undermine the credibility of another person, and ruin their reputation or otherwise be hurtful.

A good way to remember pernicious as part of your vocabulary is to mentally link it to viscious. They have meanings that may come up in similar subjects, and both sound the same; they wouldn't seem out of place in a rap. I would attempt one, but the effects could be pernicious against you. Some examples;

"Please stop these pernicious lies at once!"
"These pernicious weeds are killing the plants in my garden."
(Matter of factly in an online shooter when someone throws a grenade into a room full of you and your teammates) "Well that was pernicious"

Enjoy your vocabulary!