To move quickly with light, irregular motions, like a bird or flag moving in the wind, or to beat rapidly in an uneven way.
From Old English 'floteren,' possibly imitative in origin, meaning to move restlessly. Related to 'flit' and 'float,' all expressing quick, light movement in Germanic languages.
Flutter is one of many English words that actually sound like what they describe—'flutter,' 'flap,' 'flit'—and linguists call these onomatopoeia or 'sound-symbolic' words that let our ears literally hear the action.
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