c. 1300, usually plural, bowels, "human organs of the abdominal cavity," from late 14c. specifically as "human intestines," from Old French boele "intestines, bowels, innards" (12c., Modern French boyau), from Medieval Latin botellus "small intestine," originally "sausage," diminutive of botulus "sausage," a word borrowed from Oscan-Umbrian.
Transferred sense of "the viscera as the seat of emotions" is from late 14c.; especially "inner parts as the seat of pity or kindness," hence "tenderness, compassion." Greek splankhnon (from the same PIE root as spleen) was a word for the principal internal organs, which also were felt in ancient times to be the seat of various emotions. Greek poets, from Aeschylus down, regarded the bowels as the seat of the more violent passions such as anger and love, but by the Hebrews they were seen as the seat of tender affections, especially kindness, benevolence, and compassion. Splankhnon was used in Septuagint to translate a Hebrew word, and from thence early Bibles in English rendered it in its literal sense as bowels, which thus acquired in English a secondary meaning of "pity, compassion" (late 14c.). But in later editions the word often was translated as heart. Bowel movement is attested by 1874.