@pganssle Roboticist on Google images and you'll see that the look is basically business casual with a T-Shirt. Think "young tech company engineer" or "grad student at a university".
You might also look at what kids in robotics competitions wear. It's basically the same outfit but more colorful.
If you want to make it more distinctly a costume, you might include a lanyard with a made up ID or competition badge. You can have fun with the contents.
A hoodie might be good if it's cold.
@pganssle Hmm, I guess the "hacker" look would have the same issue.
In that case, your choices are basically mad scientist or cyborg.