"internal parts of animal bodies," c. 1300, from Old French entrailles (12c.), from Late Latin intralia "inward parts, intestines" (8c.), from altered form of Latin interanea, noun use of neuter plural of interaneus "internal, that which is within," from inter "between, among" (from PIE *enter "between, among," comparative of root *en "in").
Latin interanea yielded Late Latin intrania, hence Italian entrango, Spanish entrañas, Old French entraigne; the alternative form that led to the Modern English word evidently is from influence of the Latin neuter plural (collective) adjective suffix -alia (French -aille).
updated on May 07, 2017