The idiom "economical with the truth" originated from a statement made by Sir Robert Armstrong, a British civil servant, during the 1986 Spycatcher trial. Armstrong used the phrase to describe a situation where someone deliberately avoids telling the complete truth or provides a distorted version of the truth while trying to maintain a semblance of honesty. The idiom is used to imply that the person is selectively withholding or manipulating information to shape a narrative that serves their own interests or agenda.