: I found three other sources that use the term. Two provide minor coverage (though one is about a specific language, Malbolge), while one provides significant coverage of the topic:

  Just a brief one-line usage of the term to describe Malbolge: "Malbolge is known as one of the most esoteric programming languages. In this paper, we prove that Malbolge is weakly Turing complete."

  Just a brief one-line usage of the term in the references, citing this article: "The following URL has some links to the different Esoteric Programming Languages: http://en.wikipedia.org/wiki/Esoteric_programming_language"

  Discusses the topic at length. Largely verbatim excerpts from a 2005 conference paper co-authored by Mateas. Favors the term "weird language" but says "esoteric" is more commonly used.

 I think Mateas and Cox are enough to establish notability of the subject. As far as I can tell, the term as it's used here was invented by editor 163.1.103.122 in this article in 2002, and expanded in scope later that day by editor 213.253.40.113 (the definition has been stable since then). The term may have been taken out of context from an earlier website called "Esoteric Topics in Computer Programming"; that site used "Esoteric Programming Languages" as a section name about a much broader set of languages, seeming to use "esoteric" as an adjective like "unusual" rather than as part of a specific term. None of this is from a reliable source, it's original research and not suitable for inclusion in the article, but it may be useful for others trying to find information on the subject.