Flutter

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Old English floterian, flotorian, a frequentative form related to fleet4.

wiktionary

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From Middle English floteren, from Old English floterian, flotorian(“to float about, flutter”), from Proto-Germanic *flutrōną, frequentative of Proto-Germanic *flutōną(“to float”), equivalent to float +‎ -er(frequentative suffix). Cognate with Low German fluttern, fluddern(“to flutter”), German flittern, Dutch fladderen; also Albanian flutur(“butterfly”). More at float.

etymonline

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flutter (v.)

Old English floterian "to flutter (of birds), to fly before, flicker, float to and fro, be tossed by waves," frequentative of flotian "to float" (see float (v.)). Meaning "throw (someone) into confusion" is from 1660s. Related: Fluttered; fluttering. As a noun, "quick, irregular motion," from 1640s; meaning "state of excitement" is 1740s. Flutterpate "flighty person" is from 1894.