CJK Unified Ideograph-77F6

It means a rock or stony ledge projecting into water.

Chinese usage keeps the physical image: a waterside rock, rocky bank, or stone beach, often in fishing and place names. The character also has a verb sense for water striking rock. In a modern sentence, read it as shore rock unless the context is classical or deliberately literary.

Where did 矶 come from?

Its earliest supported record here is the traditional form 磯.

The traditional form 磯 is preserved in the Shuowen tradition submitted in 121 CE, where a later-added entry glosses it as a large rock struck by water. Its starting job was to name a water-edge feature. The opened sources identify an evolution, not a single inventor or first inscription.

The related Japanese word, written 磯 or いそ, appears in the Kojiki (712) and Man'yōshū (8th century), then names rocky shores and the coastal fishing life around them. Chinese usage also preserves a verb extension: water hitting stone, then figuratively provoking or offending someone.