poisonous

Etymology

From Middle English poisounous, poysonouse, equivalent to poison + -ous.

adj

1. Containing sufficient poison to be dangerous to touch or ingest.
   While highly poisonous to dogs, this substance is completely harmless if ingested by humans.
   Nor taint-worm ſhall infect the yeaning herds / Nor penny-graſs, nor ſpearwort's poiſ'nous leaf. 1757, John Dyer, “Book I”, in The Fleece: A Poem[…], London: R. and J. Dodsley, page 40
   I had picked a mushroom so poisonous that particles of it, stuck to my fingers and accidentally swallowed, could have made me deathly ill, and a piece the size of my thumb could have killed me. 2003, Charles L. Fergus, Common Edible and Poisonous Mushrooms of the Northeast, Stackpole Books, page 77
2. (figurative) Negative, harmful.
   He didn't want to end up like his grandfather, bitter and intractable, consumed in his hatred like an addict on haze — a poisonous attitude that would possess him all his remaining years. 2013, Kylie Griffin, Allegiance Sworn, Penguin