What the Hell is a Slug?

March 24, 2023

I hadn’t used GitHub in a while the other day and needed to download a repository for a Python tool that I wanted to use. While reading the documentation I came across a reference to the author’s “slug.” It sounded sort of derisive at first glance and my curiosity was piqued by there being a geek term out there that I hadn’t noticed before.

What I learned is that the term “slug” refers to a URL-friendly version of a string, typically used in the context of a repository or user names in URLs. It’s a way to represent a human-readable string with URL-encoded characters to ensure compatibility with web browsers and adhere to URL standards.

Understanding slugs is particularly relevant when constructing URLs for GitHub repositories, users, or organizations, in web applications, or when referencing resources on GitHub and other sites programmatically.

In the case of GitHub repositories specifically, the “slug” is the part of the URL that uniquely identifies a repository. It is derived from the repository name, with special characters replaced or encoded. For example, if you have a repository named, “My Repository for Python Code,” the corresponding slug in the full URL might be:

https://github.com/myusername/my-repository-for-python-code.

In this URL, “my-repository-for-python-code” is the slug for the repository named “My Repository for Python Code.”

In addition to repositories, user and organization names on GitHub and elsewhere can also have corresponding slugs, and slugs don’t have to have dashes in them. For example, if a user on GitHub has the real-world name John Doe and the username “JohnDoe,” the corresponding slug in the URL might be “johndoe.”

The concept of slugs is not unique to GitHub. It is commonly used in web development for creating human-readable and SEO-friendly URLs.

One of the most interesting facts about slugs, however, is how the name came to be. According to Wikipedia the term goes all the way back to the 15th century. It’s been found in Norse, Middle English, Danish, and Icelandic vocabularies referring variously to laziness, slowness, slothfulness, or sluggishness. Interestingly, we named the animal “slug” after this derisive term for people around the 18th century, and not the other way around.

The term has also referred to ammunition, a drink of alcohol, hitting someone, and a counterfeit coin. By the 1800s it came to refer to an unformed piece of raw metal, which in turn, back in the days of manual typesetting, came to refer to a group of metal letter blocks in a single strip called a stick. In the newspaper industry, the word then morphed into a term for a short name for an article that was in the production process, and the AP Stylebook called it a “slugline.” Now it becomes a little clearer how it came about as a word for a way of representing websites in URLs.

The term turns out to have over four hundred years of history, not all of it negative (in fact I love slugs and snails, although I’m also admittedly not a gardener). It has thus evolved to encompass meanings in the realms of biology, munitions, metallurgy, printing and eventually computing. Its diverse applications highlight the adaptability of human languages and the fascinating ways that words can morph and meander in meaning throughout time.