Thayer's Greek Lexicon

 1. bowels, intestines, (the heart, lungs, liver, etc.)
  1) bowels
  2) the bowels were regarded as the seat of the more violent
passions, such as anger and love; but by the Hebrews as the
seat of the tenderer affections, esp. kindness, benevolence,
compassion; hence our heart (tender mercies, affections, etc.)
  3) a heart in which mercy resides