Probably from listening to another speaker who defined it, adopting their definition. Possibly from making it up on the spot. After all, someone has to create the first definition! Words weren't handed down to us by some magical deity.

But that doesn't mean the audience came from the same place. It is very possible, and often happens, that they heard/created an entirely different definition for the same word. The speaker cannot possibly use their definition before even knowing of it.