Boiling or surging motion; the action of ebbing and flowing like a tide; agitation or heat (archaic).
From Latin 'aestuare' (to surge, boil), related to 'aestus' (tide, heat). An older noun form now replaced by 'estuary' for the geographic feature.
Esture is a wonderfully archaic word that originally meant the very thing an estuary does—that constant surging, boiling action of tides meeting river water.
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