noun

• (countable, uncountable) A type of confectionery, originally (since Ancient Egyptian times) made from this plant, but now generally made of sugar or corn syrup, gelatin that has been presoftened in water, gum arabic, flavorings, and sometimes beaten egg whites, all whipped to a spongy consistency.
  "The Disagreeable Girl is a female of the genus homo persuasion, built around a digestive apparatus that possesses marked marshmallow proclivities."
• (figuratively) Someone who is soft and benign.
  "He acts like a total badass, but he's really just a marshmallow once you get to know him."

verb

• (transitive, intransitive) To soften (literally or figuratively).
  "For i could still feel her small face in my hands, her lips marshmallowing under the brushing knuckle of my index, the hard press of her capable contoured body."