The phrase "If it doesn't fit, you must acquit" became famous during the OJ Simpson trial in 1995. It was used by Simpson's defense attorney, Johnnie Cochran, during Simpson's attempt to try on a bloody glove found at the crime scene, which was believed to have been worn by the killer. The prosecution had argued that the glove was a match to a similar glove found at Simpson's home, but when Simpson tried to put it on in court, it appeared to be too small for his hand.
Cochran used the phrase to argue that the prosecution's case was flawed, and that the evidence did not support the charges against Simpson. He suggested that the prosecution had manipulated the evidence to fit their theory of the case, and that the jury should acquit Simpson if they had any doubts about his guilt.
The phrase became a memorable and oft-cited soundbite from the trial, and has since been used in a variety of contexts to express skepticism about the strength of a case or the reliability of evidence. However, it should be noted that the use of the phrase in the Simpson trial was controversial, and many legal experts argue that it oversimplifies the role of evidence in a criminal trial, and can be misleading or even dangerous in some cases.