For those unfamiliar, when a guy and girl are walking on the sidewalk, the guy is supposed to take the street side.
This is so that, practically, if a car drives through a puddle or something, the guy will get splashed rather than the girl.
But it actually dates back to Rome, when they had walking paths through the aqueducts. There was a wall on one side and waste on the other, and it was polite to walk on the waste side.
THE MORE YOU KNOW.