Marshmallows originally came from the mallow plant, which grows in marshes. That's basically what the etymology boils down to. However, it's not that simple because the portmanteau occurred more than a thousand years ago: in the times of Old English, back when marshmallows were very different from today, the word was merscmealwe, from the Old English words mersc for "marsh" and mealwe for the plant. Mersc, through Proto-Germanic marisko, comes from the Proto-Indo-European root mori, which was designated to any body of water. Mealwe, meanwhile, actually takes an Italic route, deriving from Latin malva, meaning "mallow" as well. This has a Greek cognate, but cannot be accurately reconstructed to PIE. An obscure Mediterranean language is hypothesized as the origin. Usage of the word marshmallow steadily increased through the twentieth century due to increased commercial availability, with today having the highest utilization ever.
Adam Aleksic, a leading contender for valedictorian of his high school, is a 215-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and probably isn't spying for the Uzbek government.
The Etymology Nerd