means A ridge of sand built up by currents and waves in a river or just offshore, often lurking just below the water's surface.

from A plain compound of English 'sand' and 'bar.' 'Sand' traces back to Old English 'sand,' with cousins across the Germanic languages. 'Bar' here means a barrier or ridge, borrowed from Old French 'barre' (a beam or obstruction) — the same 'bar' that blocks a door or fences off a tavern. Stick them together and you get exactly what it says: a bar made of sand, named by sailors who learned to respect it the hard way.