Flask
Middle English (in the sense ‘cask’): from medieval Latin flasca . From the mid 16th century the word denoted a case of horn, leather, or metal for carrying gunpowder. The sense ‘glass container’ (late 17th century) was influenced by Italian fiasco, from medieval Latin flasco . Compare with flagon.
wiktionary
From Middle English flask, flaske(“case, cask, keg”), from Old English flasce, flaxe(“bottle, flask”) and Medieval Latin flascō(“bottle”); from Frankish *flasko, *flaska; whence also Dutch fles; both from Proto-Germanic *flaskǭ(“braid-covered bottle, wicker-enclosed jug”) (whence also German Low German Flaske, Fless, German Flasche), from Proto-Indo-European *ploḱ-skō(“flat”) (whence also Lithuanian plókščias, Czech ploský, Albanian flashkët).
Sense 2 from Italian fiasco and sense 3 from Middle French flasque(“powder flask”), itself from Old Spanish flasco, frasco, both from Late Latin above.
etymonline
flask (n.)
mid-14c., from Medieval Latin flasco "container, bottle," from Late Latin flasconem (nominative flasco) "bottle," which is of uncertain origin. A word common to Germanic and Romanic, but it is unclear whether the Latin or Germanic word is the original (or whether both might have got it from the Celts). Those who support a Germanic origin compare Old English flasce "flask, bottle" (which would have become modern English *flash), Old High German flaska, Middle Dutch flasce, German Flasche "bottle." If it is Germanic, the original sense might be "bottle plaited round, case bottle" (compare Old High German flechtan "to weave," Old English fleohtan "to braid, plait"), from Proto-Germanic base *fleh- (see flax).
Another theory traces the Late Latin word to a metathesis of Latin vasculum. "The assumption that the word is of Teut[onic] origin is chronologically legitimate, and presents no difficulty exc[ept] the absence of any satisfactory etymology" [OED]. The similar words in Finnish and Slavic are held to be from Germanic.
