If you're in a funk, it means that you've been feeling sad. You might be in a serious funk after your best friend moves across the country.
One way to use funk is to mean "blues" or "depression." Everyone's in a funk sometimes — for some people, the shorter, darker winter days automatically put them in a bit of a funk. Funk also refers to a bluesy kind of music with a strong underlying bass rhythm. It comes from funky, which means "stinky" or "musty smelling," from the French funkière, "smoke." In African American music, funky became a compliment, a description of soulful, earthy music — like funk itself.