The phrase has a number of variations, including 'living over the brush, and 'jumping the broomstick'. It dates back to the early slaves in southern American states. Because no-one was too bothered about slaves' living arrangements, they themselves didn't worry to much about official wedding ceremonies. If a couple wanted to set up home together, they'd either join hands over a broom and say a few rudimentary phrases, or even jump over a broomstick lying on the ground, which meant they were 'married' and could live together. The phrase has passed on into modern speech - 'living over the brush' was a common expression of disapproval in northern towns in the UK for a couple living together, but not married.