the.com/coastal shelf

means The gently sloping, submerged edge of a continent, extending from the shoreline out to the point where the seabed suddenly plunges into the deep ocean.

from A straightforward compound of practical English, not poetry: 'coastal' from Latin 'costa' meaning rib or side, borrowed into navigational language for the land bordering the sea, and 'shelf' from Old English/Germanic roots describing a flat ledge or plank-like projection — the same word you'd use for a bookshelf or a ledge of rock. Geologists and mapmakers simply stuck the two together to name the real underwater ledge that rims the continents, since it behaves exactly like a shelf: a flat surface that holds things up before an edge where everything drops away.