1
a
: causing sickness or death by entering or touching the body : containing poison
poisonous mushrooms
The leaves of the plant are poisonous. [=toxic]
Chocolate is poisonous to dogs.
b
: capable of putting poison into another animal's body by biting it
This species of spider is extremely poisonous.
poisonous snakes
2
: very harmful or unpleasant
poisonous rumors
He was in a particularly poisonous [=nasty] mood last night.
Cite this Entry
“Poisonous.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/poisonous. Accessed 18 Jun. 2026.
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