pernicious
per-NISH-usadjMeaning causing great harm, often slowly or in a way that is hard to see
Memory hook per-NICE-ious: looks NICE, does HARM. The pretty pink flowers spreading through Lin’s garden look nice, but week by week they choke out every bean and carrot; the weed is pernicious.
Gossip can have a pernicious effect on a friendship.
In context The habit seemed harmless at first, a few minutes of games before homework, but it proved pernicious, quietly eating up whole evenings until her grades began to slip.
Similar harmful, damaging, destructive, injurious
Opposite harmless, benign, beneficial
From Latin perniciosus, “ruinous,” from pernicies (ruin, destruction).