shoal n.1

[large number of fish] late 14c., scole, from Middle Dutch schole (Dutch school) "group of fish or other animals" (porpoises, whales), which is cognate with Old English scolu "band, troop, crowd of fish," both from West Germanic *skulo- (source also of Old Saxon scola "troop, multitude," West Frisian skoal), perhaps with a literal sense of "division," and from PIE root *skel- (1) "to cut." Compare shoal (n.2)), the assibilated form of the same word. For possible sense development, compare section (n.) from Latin secare "to cut."

c. 1400, schalowe, shaloue, "not deep" (of water, a river, etc.); also of the human body, "thin, emaciated," probably from the same source as Old English sceald "shallow" (see shoal (n.)), perhaps as an abbreviated form of *scealdig. Of breathing, attested from 1875; of thought or feeling, "superficial," by 1580s. Related: Shallowly; shallowness. The noun, usually shallows, "place where water is not deep" is recorded from 1570s, from the adjective.

also *kel-, Proto-Indo-European root meaning "to cut."

It forms all or part of: coulter; cutlass; half; halve; scale (n.1) "skin plates on fish or snakes;" scale (n.2) "weighing instrument;" scalene; scallop; scalp; scalpel; school (n.2) "group of fish;" sculpture; shale; sheldrake; shelf; shell; shield; shoal (n.2) "large number;" skoal; skill.

It is the hypothetical source of/evidence for its existence is provided by: Latin culter "knife," scalpere "to cut, scrape;" Old Church Slavonic skolika "mussel, shell," Russian skala "rind, bark," Lithuanian skelti "split," Old English scell "shell," scalu "drinking cup, bowl, scale of a balance."