This metaphorical expression is commonplace in English language. If you want to find out about its origin as well as its meaning and usage then read below!
The expression is used to denote the act of discerning a meaning of something which isn’t made clear. It relates to the act of figuring out something which is not made explicit in a text. It can equally be used in relation to other forms of communication such as “reading between the lines” in a speech made by someone due to the fact that it is used in a metaphorical sense.
The expression derives from the mid 19th century in cryptology to denote the act of hiding extra meanings in between the main lines of a message. Shortly after this the expression came into usage in day-to-day and general conversation, in a far less literal sense.
The first written recorded use of the expression can be found in an 1862 edition of The New York Times which reads “Earl Russell’s dispatch does not recite the terms of the 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.”
An example of the usage of this expression can be found in a quote by author and philosopher Shannon L. Alder. She is quoted to have said “the most important thing in communication is hearing what isn’t being said. The art of reading between the lines is a lifelong quest of the wise.”