“Fruit of The Poisonous Tree” is a famous phrase in constitutional criminal law. It refers to the suppression of unlawfully obtained evidence. Unconstitutional police conduct is the tree, and the evidence the police derived from the conduct is the fruit. Imagine that the police enter a home without probable cause or a warrant (unlawful conduct) and after a search of the house, find a cache of unlawful assault rifles (fruit). The owner of the house would not have to go to trial for possession of the guns because they would be suppressed and excluded from evidence due to the police conduct.
The phrase was first used by the United States Supreme Court in a case called Nardone v. United States all the way back in 1939. The High Court reversed Mr. Nardone’s tax fraud case TWICE, once due to the trial court’s refusal to exclude evidence of an illegal wiretap and a second time for the trial court’s failure to allow the defense to inquire into what other evidence came from the illegal wiretap.
You see, once the police break the law, all of the evidence that comes from that lawbreaking must be suppressed. If the police illegally search my home and find drugs, and this leads me to confess to the police right away that the drugs were mine, then both the drugs and the confession must be suppressed.