undefiniert – undefined
What does “undefined” mean?
The un- is a prefix that means “not”, so then undefinert is nicht definiert (not defined).
Definiert is the past participle of the verb definieren, which comes from the Latin definire, according to Duden.
When I tried to dig out more of the history by Googling, typing “definire” tended to give me Italian rather than Latin, so here’s what I found about the Italian definire from the Italian Etymology Dictionary (yes, it also says it’s from Latin):
De is an intensifier, while finire means “to end”, “to determine”, “to limit”.
The verb finire comes from the word finis – something that has ended, and is, in essence, complete.
So, something that is defined is something that is complete, since if it isn’t complete you can’t really know what it is, and you can’t describe it.
Yet a return value, in general, signals that the function has finished execution, and that means that it is complete.