Is it possibly: 맛이 없어? An informal way of saying "It doesn't taste good." "It has little flavor."

I guess you mean 맙소사! (mapsosa!) which is kind of like "Oh my god!" or "I can't believe it!" I have only ever heard this word on TV, I have never heard another Korean use it in conversation and I don't think I've ever used it myself. It's most commonly used in written Korean.