The term 'Marshmallow Birdie' is a combination of two crossword solutions: PEEP (4 letters) and MARSHMALLOW (10 letters). PEEP is a verb meaning to make a soft, high-pitched sound, usually associated with birds. Marshmallow is a type of confectionery made from sugar, corn syrup, and gelatin that is often used in desserts. Together, the two words form a whimsical phrase that can be used to describe a bird that makes a soft, high-pitched sound, like a marshmallow.