1.  pestilent(s = adj.all) baneful, deadly, pernicious - exceedingly harmful;
Derived form noun pestilence₃

2.  pestilent(s = adj.all) pestiferous, pestilential, plaguey - likely to spread and cause an epidemic disease; "a pestilential malignancy in the air"; "plaguey fevers"
Derived form noun pestilence₂

   Pestilential; noxious; pernicious; mischievous. Shak. [1913 Webster]

pestilent, adj.
1 destructive to life, deadly.
2 harmful or morally destructive.
3 colloq. troublesome; annoying.

Derivative:
pestilently adv.

Etymology:
L pestilens, pestilentus f. pestis plague

N  insalubrity, unhealthiness, nonnaturals, plague spot, malaria, death in the pot, contagion, toxicity, insalubrious, unhealthy, unwholesome, noxious, noisome, morbific, morbiferous, mephitic, septic, azotic, deleterious, pestilent, pestiferous, pestilential, virulent, venomous, envenomed, poisonous, toxic, toxiferous, teratogenic, narcotic, contagious, infectious, catching, taking, epidemic, zymotic, epizootic, innutritious, indigestible, ungenial, uncongenial, deadly.