Wow, the OED is not so clear on the origin, either!
demijohn, n.
Pronunciation:
Brit.
/ˈdɛmɪdʒɒn/
,
U.S.
/ˈdɛmiˌdʒɑn/
Forms: 17
demijan, 18
demijean,
demi-john,
demijohn.
Frequency (in current use):
Etymology: In French dame-jeanne (1694 Th. Corneille dame-jane, 1701 Furetière Dame Jeanne, lit. ‘Dame Jane’); so Spanish dama-juana(as if Dama Juana); modern Provençal, in different dialects, dama-jana, damajano, damojano, damejano, dabajano, debajano; Catalan damajana; Italian damigiana; modern Arabic damajānaħ, dāmajānaħ, etc. in 19th cent. lexicons.
The current English form is the result of popular perversion as in ‘sparrow-grass’; the earlier demijan, demijean, approach more closely to the French and Romanic, whence the word was adopted. The original nationality and etymology of the word are disputed: see Rev. A. L. Mayhew in Academy 14 Oct. 1893. Some have assumed the Arabic to be the source of the Romanic forms, and have sought to explain this as of Persian origin, and derived from the name of the town Damghān or Damaghān, a commercial emporium S.E. of the Caspian. But this is not supported by any historical evidence; moreover, the word does not occur in Persian dictionaries, nor in Arabic lexicons before the 19th cent., and the unfixedness of its form (dāmijānaħ, dāmajānaħ, damajānaħ, damanjānaħ) points, in the opinion of Arabic scholars, to its recent adoption from some foreign language, probably from Levantine use of Italian damigiana. Assuming the word to be Romanic, some have taken the Provençal and Catalan forms as the starting-point, and conjectured for these either a Latin type *dīmidiāna from dīmidium half (Alart in Rev. Lang. Rom. Jan. 1877), or the phrase dē mediāna of middle or mean (size) (in illustration of which Darmesteter cites from a 13th cent. tariff of Narbonne the phrase ‘ampolas de mieja megeira’ = Latin ampullās dē mediā mensūrā). But these suggestions fail to explain the initial da- prevalent in all the languages; on account of which M. Paul Meyer (like Littré) thinks that all the Romanic forms are simply adaptations or transliterations of the French, this being simply Dame Jeanne ‘Dame Jane’, as a popular appellation (compare Bellarmine, greybeard, etc.). This is also most in accordance with the historical evidence at present known, since the word occurs in French in the 17th cent., while no trace of it equally early has been found elsewhere.