What part of the human body gets its name from the Latin for 'twelve' or 'twelve each', a reference to the length of twelve fingers ?

If I remember correctly, the Latin for twelve is something like duodecim. So it's the duodenum?

Duodenum it is. Its full name was 'intestinum duodenum digitorum, 'the intestine of twelve fingers together', because it was twelve finger-lengths long.(And, yes, the Latin for the number twelve is duodecim )