"full of venom, noxious or hurtful by means of venom," c. 1300, from Anglo-French venimeus, Old French venimos (12c., Modern French venimeux), from venim (see venom). Earliest recorded use is figurative; literal sense by early 14c. Biologists have tended to preserve a distinction between venomous and poisonous that the general language has forgotten: venomous is applied to what bites or stings to inject toxins, poisonous to what unload toxins when eaten. Related: Venomously; venomousness.
legendary poisonous tree of Java, 1783, via Dutch, from Malay (Austronesian) upas "poison," in pohun upas "poison tree." As the name of an actual tree (Antiaris toxicaria) yielding poisonous sap, from 1814.
c. 1400, in reference to wounds, ulcers, etc., "full of corrupt or poisonous matter," from Latin virulentus "poisonous," from virus "poison" (see virus). Figurative sense of "violent, spiteful" is attested from c. 1600. Related: Virulently.
late 14c., apparently just what it looks like: a fanciful name from Middle English tadde "toad" (see toad) + stole "stool" (see stool). Toads themselves were regarded as highly poisonous, and this word is "popularly restricted to poisonous or inedible fungi, as distinct from edible "mushrooms" [OED]. Compare toad-cheese, a poisonous fungi; toad's meat (1886), a "rustic" term for toadstool.