According to Wikipedia, the term dates back to at least 2002: > The frequent use of a double underscores in internal identifiers in Python gave rise to the abb

According to Wikipedia, the term dates back to at least 2002: > The frequent use of a double underscores in internal identifiers in Python gave rise to the abbreviation dunder; this was coined by Mark Jackson[3] and independently by Tim Hochberg,[4] within minutes of each other, both in reply to the same question in 2002.[5][6]