Dave Plowman (News) wrote:

Thinking on it's probably rubbish since it would mean an internal and
external thread on the same elbow - and I've not seen this on any old
barrel work. Unless a *real* expert can confirm it.

Yes, steel/iron street elbows exist.

http://www.bes.ltd.uk/products/160a.asp

Don't know if that's the origin of the term; I read the same
explanation on a US-based forum.