The phrase to toe the line means to accept the authority, policies or principles of a particular group, especially unwillingly.

Its literal sense is to stand or crouch with the toes touching the line, especially at the start of a race or fight. The current meaning is an extension of a figurative usage, to present oneself in readiness for a race, fight, contest or undertaking, which developed from this literal sense.

Other words than line were also formerly used: mark, crack, trig (a line marked out on the ground, at which a player at bowls, quoits, etc., stands, or from which runners start in a race) and scratch (a line drawn as an indication of a boundary or starting-point – in prize-fighting, the line drawn across the ring, to which boxers were brought for an encounter – probably so named because it was originally scratched on the ground).

A description of the procedure leading to toeing the mark in readiness for a fight is found in Two Years Before the Mast. A Personal Narrative of Life at Sea (1840), by the American author, lawyer and politician Richard Henry Dana Jr. (1815-82):

(edition: Harper & Brothers, New York, 1842)
A broad-backed, big-headed Cape Cod boy, about sixteen years old, had been playing the bully, for the whole voyage, over a slender, delicate-looking boy, from one of the Boston schools, and over whom he had much the advantage, in strength, age, and experience in the ship’s duty, for this was the first time the Boston boy had been on salt water. The latter, however, had “picked up his crumbs,” was learning his duty, and getting strength and confidence daily; and began to assert his rights against his oppressor. Still, the other was his master, and, by his superior strength, always tackled with him and threw him down. One afternoon, before we were turned-to, these boys got into a violent squabble in the between-decks, when George (the Boston boy) said he would fight Nat, if he could have fair play. The chief mate heard the noise, dove down the hatchway, hauled them both up on deck, and told them to shake hands and have no more trouble for the voyage, or else they should fight till one gave in for beaten. Finding neither willing to make an offer of reconciliation, he called all hands up, (for the captain was ashore, and he could do as he chose aboard,) ranged the crew in the waist, marked a line on the deck, brought the two boys up to it, making them “toe the mark;” then made the bight of a rope fast to a belaying pin, and stretched it across the deck, bringing it just above their waists. “No striking below the rope!” And there they stood, one on each side of it, face to face, and went at it like two game-cocks.

The phrase is also used figuratively in the same book:

Captain T— was a vigorous, energetic fellow. As sailors say, “he hadn’t a lazy bone in him.” He was made of steel and whalebone. He was a man to “toe the mark,” and to make every one else step up to it.

And, after savagely flogging a sailor, the captain declares to the crew:

“You’ve been mistaken in me—you didn’t know what I was. Now you know what I am!”—“I’ll make you toe the mark, every soul of you, or I’ll flog you all, fore and aft, from the boy, up!”

The earliest instance of the current meaning that I have found is from The Connecticut Courant (Hartford, Connecticut) of 8^th February 1804; it published the account that Robert Skinner gave on 19^th December 1803 of events that had taken place in November 1799:

I […] went over to Mr. Tracy’s, found Gen. Tracy in his Office, together with my Father Gen. Skinner, Parson Huntington, and James Gould. As I entered the room, my Father had just rose from his seat and was about going. I immediately discovered from something that was said, (what, I do not distinctly recollect,) that Gen. Tracy and my Father had been warmly engaged in a political dispute, which made a strong impression on my mind, (as I was then a federalist) knowing that my father entertained suspicions of the Federal administration, and being well satisfied that no one else present before, that evening, had knowledge of the fact, I regretted that he should have on that [illegible] [illegible] his feelings on the side of a question at that time so odious; my Father soon took leave of Gen. Tracy […]. Soon after this Mr. Allen returned from Doct. Smith’s to Gen. Tracy’s, took his seat by me. The first word that was uttered after he came in was by Gen. Tracy to Mr. Allen, “Gen. Skinner does not seem to toe the mark with us.”

 

In the Oxford English Dictionary (2^nd edition – 1989), the earliest quotation for the verb toe in the sense to touch or reach with the toes is from The Diverting History of John Bull and Brother Jonathan (first published in 1812), written by the American author James Kirke Paulding (1778-1860) under the pseudonym of Hector Bull-Us:

He began to think it was high time to toe the mark.

Many ‘etymologists’ have affirmed that in this quotation to toe the mark “already had the modern figurative sense of conforming to the usual standards or rules”.

But the context in which this quotation appears contradicts this affirmation (“the Squire” is John Bull):

(3^rd edition: M. Carey & Son, Philadelphia, 1819)
The patience of honest Jonathan, too, was now worn quite threadbare, and he began to think it was high time to toe the mark, and try to put an end to the Squire’s troublesome pretensions.

Here, to toe the mark means to make oneself ready for a fight; in a subsequent chapter, we read:

Jonathan having at last persuaded his wife to let him have a bout with John Bull, gathered himself together, and wrote the Squire a mortal defiance; in which […] he gave Bull fair notice that he and his tenants meant, thenceforward, to try what the great statute of club-law would do for them. Let Squire Bull then come out like a man and fight him in fair battle if he dared.

In the same chapter, the author writes that “some of Jonathan’s overseers put up their sneakers, and wouldn’t toe the mark” to mean that they sneaked away, and would not enter the fight.

As the phrase is, for many, of obscure origin, it has popularly been interpreted as to tow the line, the image being to pull on a line in the sense of a rope. For example, the following is from The Guardian Of The Misplaced Realm (2012), by Pauline Drummie:

Diana gave me an ultimatum; either I had a word with Sammy and asked him to behave at school or he wasn’t to attend anymore [sic] lessons. So, like any good dad, I started teaching Sammy myself. I didn’t want my son to be one of those kids who always tows the line.