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"poisonous" is also used as a crossword clue. If that's what you saw, the answer is often toxic (5 letters).
Definitions
Able to harm or kill by poison.
Word origin
From Middle English poisounous, poysonouse. By surface analysis, poison + -ous.
Used in a sentence
“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.”
“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.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Toxic”9 letters
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