In 2022, Vitalik explained a habit of his: when he is writing and wants to find his place again later, he types the word moose and searches for it. It almost never turns up by accident, so it makes a perfect marker.

He has history with the word — two tweets, one in 2017 and one in 2018, that say nothing but “Moose.”

So here is our best guess. In 2016, someone wanted a placeholder for a function they would name properly later. They typed moose(), shipped it, and never came back. We can't prove it — but it beats anything we could make up, and you can go read it yourself.