The word アルコール is very similar to the Portuguese alcohol (pronounced alcōl), this seems closer than the English アルコホール. So is the real etymology Portuguese rather than English?
Surveying various resources, here's what I've found:
- Shogakukan's 国語大辞典 states it's from either Dutch or English alcohol (my local copy: "（オランダ・英alcohol）")
- Different editions of Daijirin state either Dutch definitively (my local copy: "アルコール  【(ｵﾗﾝﾀﾞ) alcohol】") or possibly English (third edition, online here: "アルコール .【alcohol】" -- implying English if no other language is specified).
- Daijisen states Dutch or English ("アルコール（〈オランダ〉・〈英〉alcohol）").
- The fifth edition of the Shinmeikai also states Dutch (my local copy: "（オ alcohol）", with the オ short for オランダ).
- The Japanese Wikipedia article's section on the etymology of this term states a source from Dutch in the Edo period (1600-1868), and notes that there used to be an
/h/in there much as in Dutch or English, but that historical kana usage and pronunciation changes led this ホ to become just オ, yielding the アルコール pronunciation of today.
So ultimately, the emerging consensus is that アルコール is not from English, but rather from Dutch alcohol, with the
/h/ dropping out over time due to normal Japanese historical processes. We know that both the Portuguese and Dutch were very active in trading with Japan prior to and (for the Dutch) during the Edo period. In light of the historical record, English can be ruled out, but Portuguese could still be another possible source.
アルコール  【オランダ alcohol】〈「亜爾箇児」と当てた〉
so it seems to be from Dutch (although unlike Portuguese, Dutch does pronounce the H).