To flutter, flap, or wave loosely, especially used in Scottish dialects to describe things moving with a whipping motion.
From Scottish/Northern English dialect, possibly imitative in origin, related to Old Norse 'flöfra' meaning to flutter; used since at least the 15th century in regional British English.
Scottish fisher folk used 'flaff' to describe how fish fins and sails would move in wind and water—it's one of those dialect words that captures a very specific physical sensation in a way standard English can't.
Complete word intelligence in one call. Free tier — 50 lookups/day.