The term “dox” also spelt “doxx”, and short for “[dropping] documents” first came into vogue as a verb around a decade ago, referring to malicious hackers’ habit of collecting personal and private information, including home addresses and national identity numbers. The data are often released publicly against a person’s wishes. It is a practice frowned upon by users of Reddit, a popular online forum, and many others.
via The Economist explains: What doxxing is, and why it matters | The Economist.