"Toe the line" is corruption of "Toe the Lion", a cruel practice used in old Roman times in the Colliseum.

The middle 2 toes of the rear paws of captive lions used in the games were removed. This helped control the giant felines, making it impossible for the lions to get purchase to leap out of the ring and into the stands to menace the spectators. Such an occurance might have caused a bit of unpopularity of the then-current Caesar and so all lions were "toed" before being set loose in the arena.

Since so many urban myths are being propogated here, I thought I'd start one of my own.