Here are some possible origins:
* Folklore: It's possible the idiom arose from observations of people who seemingly didn't listen or remember what was said to them, as if the information went in one ear and out the other.
* Religious symbolism: The phrase might have been influenced by biblical stories, where ears represented receptiveness to God's word. A person who ignores God's message is seen as having it enter one ear and exit the other without taking it to heart.
* Physical anatomy: The phrase might have been used figuratively, playing on the physical act of sound entering one ear and leaving the other. This emphasizes the lack of processing or understanding, as if the message just passes through without being retained.
Examples of early usage:
* 1546: In a book titled "The Pathway to the Holy Scripture," the phrase is used: "They have their ears open, but their hearts shut, so that all that they hear goeth in at one ear and out at the other."
* 1621: In a play called "The Witch of Edmonton," a character says: "I speak to her, but it goes in at one ear and out at the other."
While the exact origin may be debated, the idiom "in one ear and out the other" has been used for centuries to express the idea of someone not paying attention or failing to retain information.