To read between the lines is to find the meaning that is hinted but never said. Read between the lines, if you read between the lines, reading between the lines.
It is what everybody does with a cautious reference or an official statement. The phrase comes from notes written between the lines of a text, which carried what the text itself did not say.
1phraseread between the linesTo find the meaning that is hinted but never said.
“Her letter sounded cheerful, but if you read between the lines, she was lonely.”
read between the lines is an interpreting.
Probably from glosses written between the lines of a text, carrying what the text did not state.
Probably from glosses and commentaries which were written between the lines and provided the reader with additional information not explicitly stated in the text.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms