Something that's envenomed is poisonous on purpose, like a poison dart.
Envenomed comes from the Old French envenimer, "to poison or taint," and its Latin root venenum, "poison." This word is mostly used for things that are deliberately covered or treated with poison, such as weapons. In Shakespeare's Hamlet, Laertes is accidentally killed by his own envenomed sword, which he had dipped in poison himself. Figuratively, envenomed means "made bitter or hateful," like a person who's been envenomed by frequent bullying.