Something faint has no courage or vigor. If you suddenly round the corner and see a gang of bullies staring straight at you, chances are you'll feel faint.
Faint comes from the French feindre, for being cowardly or shirking duties. We usually use it to mean without great strength. If you faint, you pass out, from low blood supply to your brain. If you feel faint, you feel weak. Faint also means dim or vague. If you're staring out to sea, you might see the faint outline of an approaching ship. If you really can't remember the answer to something, say you "haven't the faintest idea."