What is the original meaning of 匈 (besides "chest/breast/thorax")?
匈 is the ancient character of 胸, means chest/breast. And I can't find any other original meaning of it besides this.


  “胸”的古字 [bosom;chest]
  匈,膺也。——《说文》。字亦作胷、作胸


According to the explanation of 胸, it's a later character for 匈.


  从肉,匈声。本作“匈”,胸是后起字。


匈奴 is just the transliteration from the name of an ancient nationality in North China, has nothing to do with fierce slaves.

Reference:
匈的解释
胸的解释
孙健：说说“匈奴”这个词的起源