I wonder when it became common, since I did some Python stuff many years ago and never encountered the term in that context.
On the other hand... https://en.wikipedia.org/wiki/Dunder
On the other hand... https://en.wikipedia.org/wiki/Dunder
> 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]