A harshmallow is a rare combination of two food items to create one, a common practice amongst high people.
A harshmallow is a marshmallow dipped in hummus.
This particular combination was discovered by a female, but the term was coined by a member of the male species because everyone knows girls are stupid.
girl: Mannnnnnnn.... you know what would reallyyyyyy hit the spot right now...... some marshmallow dipped in hummus.
MAN: ... Oh... a harshmallow?
girl: yaaaaaa..... wait whaaaaauuut?