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"place of shallow water," c.1300, from Old English schealde (adj.), from sceald "shallow," from Proto-Germanic *skala- (cf. Swedish skäll "thin;" Low German schol, Frisian skol "not deep"), of uncertain origin. The terminal -d was dropped 16c.
"large number" (especially of fish), 1570s, apparently identical with Old English scolu "band, troop, crowd of fish" (see school (n.2)); but perhaps rather a 16c. adoption of cognate Middle Dutch schole.
"assemble in a multitude," c.1600, from shoal (n.2). Related: Shoaled; shoaling.
accumulation of sediment in a river channel or on a continental shelf that is potentially dangerous to ships. On the continental shelf it is conventionally taken to be less than 10 m (33 feet) below water level at low tide. Shoals are formed by essentially the same factors that produce offshore bars. See sandbar.