This is based on the activities of Rob Ford, the mayor of Toronto, Canada. If a person says, 'I plead the Ford", it means they are excused from their behavior and are not obliged to explain themselves because they were smoking crack cocaine and in a drunken stupor.
In a United States court case, to cite one's own Constitutional right in the Fifth Amendment to "remain silent" as a defendant, or to not be forced to testify against one's self. This is why a lawyer cannot simply force a defendant into a confession while under oath.
Pleading the 5th means to not self-incriminate yourself, but it has the connotation of meaning that you did it.
The reason for this connotation is because if you didn't do it, why wouldn't youjust say no?
It's a way of not saying "yes you did something" but hinting that you did, but they can't really say you hinted at it.
v. To decline to give self-incriminating information. Refers to the fifth amendment of the US Constitution, which states that no citizen "shall be compelled in any criminal case to be a witness against himself..."