1. To really enjoy the entire experience of a delicious meal or food. 2. When a food or a meal is so yummy that it makes one feel very very happy. 3. When on really enjoys the flavor of a food or meal. 4. To be overtaken by flavor.
I just had the most marvelous burrito for lunch! It was a total yumfuck!" or "Wasn't that casserole a real yumfuck?!" or "Thanks for these delectable brownies. They are yumfucking me!