endo n. 1. a bicycling accident in which the rider is thrown forward over the handlebars. 2. an intentional bicycle stunt which lifts the bike’s rear tire off the ground. Etymological Note: Perhaps from end-over-end. (source: Double-Tongued Dictionary)
Further reading
When a teenager went a week without talking as part of a school project, he noticed a surprising side effect: Instead of rehearsing a response to what other people were saying to him, he was focused on listening — and feeling smarter as a result...
Among ham radio enthusiasts, the word Elmer has become an affectionate term for an experienced ham radio operator who mentors a novice. It’s also used as a verb: Elmering someone means helping a new ham. The term was coined in a 1971 magazine...
This word also refers to a trick performed on a bicycle or motorcycle in which the rider balances on the front wheel, with the rear wheel in the air.
You’re right. I’m working on the second sense for this word; I think the first cite above is unclear, and probably belongs to your definition of an endo, and not to the one I have here.