Fruit of the poisonous tree is a legal metaphor used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well.

n. in criminal law, the doctrine that evidence discovered due to information found through illegal search or other unconstitutional means (such as a forced confession) may not be introduced by a prosecutor. The theory is that the tree (original illegal evidence) is poisoned and thus taints what grows from it. For example, as part of a coerced admission made without giving a prime suspect the so-called "Miranda warnings" (statement of rights, including the right to remain silent and what he/she says will be used against them), the suspect tells the police the location of stolen property. Since the admission cannot be introduced as evidence in trial, neither can the stolen property. See also: Miranda warning

Plants that produce toxins are referred to as poisonous plants. Plants that cause irritation on contact are also described as "poisonous".