“Toe the line” means either to conform to a rule or standard, or to stand poised at the starting line in a footrace. Other phrases which were once used in the early 1800s and have the same meaning were toe the mark and toe the plank.
The most likely origin of the term goes back to the wooden decked ships of the British Royal Navy during the late 17th or early 18th century. Barefooted seamen had to stand at attention for inspection and had to line up on deck along the seams of the wooden planks, hence to “toe the line”.
On some military parade-grounds there are white lines marked, along which soldiers form up, with their toes just touching the line.
Over the years the term has been attributed to sports, including toeing the starting line in track events and toeing a center line in boxing, where boxers were instructed to line up on either side of to start a match. However, the earlier boxing term was toeing the scratch, referring to a scratch mark on the floor. One of the earliest references related to an English prize fight in 1840