guts n.

Old English guttas (plural) "bowels, entrails," literally "a channel," related to geotan "to pour," from Proto-Germanic *gut-, from PIE root *gheu- "to pour." Related to Middle Dutch gote, Dutch goot, German Gosse "gutter, drain," Middle English gote "channel, stream." Meaning "abdomen, belly" is from late 14c. Meaning "narrow passage in a body of water" is from 1530s. Meaning "easy college course" is student slang from 1916, probably from obsolete slang sense of "feast" (the connecting notion is "something that one can eat up"). Sense of "inside contents of anything" (usually plural) is from 1570s. To hate (someone's) guts is first attested 1918. The notion of the intestines as a seat of emotions is ancient (see bowel) and probably explains expressions such as gut reaction (1963), gut feeling (by 1970), and compare guts. Gut check attested by 1976.

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.

The 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.