The first time I say it out loud to someone there's often a request to hear it again. If they're a native speaker it generally clicks with an emphasis on "Thwack." If they're not, spelling it clears it up rather quickly. Usually it's because they've never heard the word before.

Once people get it they repeat it, likely because it's an entertaining and weird thing to say.

I agree with the advice that names should be easy to spell, but I also think it depends on context. Nearly all of our customer acquisition starts visually, either through the site, banners, or press, so it hasn't really been a problem.

Who knows if we'll keep it forever, but it's working for now.