There are some real odd names for functions in mathematics, but Minkowski's question mark function, denoted by $?(x)$, may be the oddest one I have ever seen.
In Zur Geometrie der Zahlen, Minkowski doesn't discuss his choice of notation (at least, not in the relevant sections that I could find and use a translator on).
Why is the function denoted by $?(x)$? Most related graphs of it (I've played around with some) fail to show anything like a question mark.