duodenum | Etymology of duodenum by etymonline

"first portion of the small intestine," late 14c., also duodene, from Medieval Latin duodenum digitorium "space of twelve digits," from Latin duodeni "twelve each" (from duodecim "twelve;" see dozen). Coined by Gerard of Cremona (d. 1187) in "Canon Avicennae," a loan-translation of Greek dodekadaktylon, literally "twelve fingers long." The intestine part was so called by Greek physician Herophilus (c. 353-280 B.C.E.) for its length, which is about equal to the breadth of twelve fingers. The classical plural is duodena.