Read between the lines
This is a very common expression which has been in use since the 19th century.
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The expression was derived from a form of cryptography - the art of writing or solving codes.
When people had to send secret information to others which they did not want third parties to read, they adopted the practice of hiding the secret information in another written message.
How did they do that?
They wrote the ordinary text in visible ink, and between the lines of the ordinary, open text, they wrote the secret information in invisible ink.
That way, an ordinary person who came across the page or paper on which the secret message had been written saw only the ordinary text, since the secret message was written in invisible ink.
Thus the secret message could be obtained or read by only those who were discerning enough to “read between the lines”, as it were, where the “real” message was hidden.
Anybody who just read the text on the paper would miss what was hidden between the lines.
From that practice of hiding secret messages between the lines of text has come this expression read between the lines, which means to understand something from a situation, a statement, etc which is not actually stated.
In other words, when you read between the lines, you try to understand what someone implies but doesn’t openly state when he or she says or writes something. That is to say you discern a meaning which isn’t made obvious or explicit.
Example:
When I asked Little Joojo why he was crying, he said there was nothing, but reading between the lines, I realised he had lost his toy and wanted a replacement.
The first example of the expression in print is from The New York Times, August 1862:
Earl Russell’s dispatch does not recite the terms of note to which it is a reply, the letter assumes a somewhat enigmatical character, and the only resource we have is, as best we may, to ‘read between the lines’ of this puzzling, but important, communication of the British Foreign Secretary.