If someone tells you you have a sagging spirit, that means your life-force seems to be missing.
Spirit comes from the Latin word for "breath," and like breath, spirit is considered a fundamental part of being alive. Some people think of the spirit as a presence that's separate from the body, which is why spirit is another word for ghost. We also use spirit to mean "the general mood or intent," like when you tell your former enemy, "I approach you in the spirit of kindness."