means A large group of fish swimming together, or a shallow sandy stretch of water where a ship might run aground.

from Two senses converge on one word. The 'group of fish' sense comes from Old English 'scolu,' a troop or multitude, a cousin of Middle Dutch 'schole' (which also gave us 'school' of fish). The 'shallow water' sense traces to Old English 'sceald,' meaning shallow, related to words across the Germanic family. Two separate roots that drifted into the same spelling — a linguistic shoal of their own.