Did you know that the phrase is toe the line, and not tow the line? It’s a very common mistake, largely because to toe isn’t commonly used a verb in English, whereas to tow can be found more frequently. Plus, you can tow a line, but what does toeing a line even mean?
The origins of the phrase are unknown, but it seems likely to have a sporting origin, as it can still be found used today to refer to athletes waiting at the start line of a race. It’s common meaning now, to conform to a convention (e.g. toe the party line in politics) is based on the image of people all lining up together in a neat line, like runners before a race.
But like I said, tow the line persists, largely because it probably seems to make sense to most people. It’s easy to imagine a tug-of-war team, for example, succeeding because everyone towed the line together.
The best thing about being wrong about toe the line is of course that most people would probably never notice you’re wrong. Even if they know it should be toe the line, you’re most likely to use it when speaking.
Tow the line is an example of an eggcorn, the mistaken replacement of a word in an expression with a word that sounds identical, and might also plausibly make sense in the context of the phrase. Like old-timer’s disease instead of Alzheimer’s disease. An eggcorn being distinct from a malapropism and a mondegreen, though mondegreens are often also eggcorns. The fact that we have so many words in English for making mistakes with words shows just how easy it is to get things wrong when using this delightfully complex language, so you shouldn’t worry about making a mistake. As the greatest philosophers have often pondered: if someone makes a mistake and no-one notices, have they made a mistake at all?