shoal
n.
1
"place of shallow water," c.1300, from Old English schealde (adj.), from sceald "shallow," from Proto-Germanic *skala- (cognates: Swedish skäll "thin;" Low German schol, Frisian skol "not deep"), of uncertain origin. The terminal -d was dropped 16c.
2
"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.
v.
"assemble in a multitude," c.1600, from shoal, n.2. Related: Shoaled; shoaling.