This is a 15th century English adage that means that children (especially girls at first) should be well-behaved and quiet.

It was once thought that children didn’t say much worth listening to and, therefore, should be silent, that this was the meaning of this adage. Children were viewed with disdain.

However, the meaning of this expression evolved into “Children should not interrupt adult conversation when in a room full of adults”. This meaning was not so dismissive as the original meaning, but was meant to teach children to respect their elders, listen, and learn. Regardless, it still has a terrible reputation of being a sign of disregard for children in the United States that still persists today.