"lower part of the alimentary canal," early 15c., from Middle French intestin (14c.) or directly from Latin intestinum "a gut," in plural (intestina), "intestines, bowels," noun use of neuter of adjective intestinus "inward, internal," from intus "within, on the inside," from PIE *entos, suffixed form of root *en "in."
Distinction of large and small intestines in Middle English was made under the terms gross and subtle. Intestine also was used as an adjective in English 16c.-19c. with a sense (as in French) of "internal, domestic, civil."