pestilent adj.

late 14c., "contaminated with dangerous disease; deadly, poisonous," from Latin pestilentem (nominative pestilens) "infected, unhealthy," from pestilis "of the nature of a plague," from pestis "deadly contagious disease" (see pest (n.)). Transferred sense of "mischievous, pernicious, hurtful to health or morals" is from 1510s; weakened sense of "troublesome" is from 1590s. Related: Pestilently.