When two people of the
opposite sex like each other very much and can see a bright future together are talking about nothing in particular just making
conversation, and
one of them interrupts and just says "You're such a Nummyface." Usually this word will cause the interrupted person to just smile really big, and start flirting more with you.
Girl: "So I was at the mall and was looking at some new
lingerie line from
Victoria's Secret and imagining myself in the sexy outfits....."
Guy: "You're such a Nummyface"
Girl: "No,
actually, you sir are the Nummyface"