If you're looking for user-deprecating names, chattel comes to mind.

As for Mercury, that's in use in plenty of places, plus it's kind of overworked. (Plus, Mercury was plenty of things besides being a messenger for the gods, including being the patron of thieves and liars and whatnot.)

Actually, that makes it more appropriate for a chat program, doesn't it? Hm. What's the Roman name for Mercury? Hermes, I think?

And if you're not concerned about the truly tacky connotations...why not BatChat? Da-na-da-na-da-na-da-na-da-na-da-na-da-na-BatChat!

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You are what you think.