Barry Finn "Toeing the line"
As I heard it - is a boxing reference when fighting to the Queensbury rules. Opponents had to meet at a line scratched in the dirt and to ensure that the fists were out of reach until the official start the pugilists had face each other one leg stretched in front of the other with a toe to the line.
Interesting because the Navy obviously predates the Marquis but which explanation came first?

either way it seems to be an obedience to rules thingy.