Summary: This idiom originated from the practice of simple steganography, where hidden messages were written in invisible ink between lines of visible text.
Deciphering the unspoken requires an analytical eye. The phrase finds its roots in 19th-century military and diplomatic correspondence. Cryptographers would often use heat-sensitive liquids or lemon juice to write sensitive information between the standard lines of a letter. Today, it serves as a metaphor for social intelligence—the ability to interpret subtext and identify nuances that are not explicitly stated in professional or personal discourse.