The phrase "hnnng" is a form of onomatopoeia used in context when a person sees a very sexually attractive person of the other
gender, or same. I personally like to think of it as the
sound a young woman makes just as she feels her ovaries are going to burst from being over exposed to excessive sexual hotness. It helps to
sound it out. "hnnng-ahhh" I like to add the gasp at the end for I know that's what those fine young mistresses do when they see I have walked into the club.
Girl 1: "omg look that sexy
guy from school walked in!"
Girl 2 turns in dramatically
slow fashion, and locks eyes with Sexy
Guy.
Girl 2: "Hnnng"
Girl 1 must now proceed to fan Girl 2 as she recovers from the sexual trauma her ovaries just endured.