means A small castle or other structure molded out of damp sand, typically built on a beach for fun and destined to be flattened by the tide.

from A transparent English compound of 'sand' and 'castle.' 'Sand' is an old Germanic word, with cousins in German 'Sand' and Dutch 'zand,' reaching back to a Proto-Germanic root for the gritty stuff. 'Castle' came into English from Old North French 'castel,' itself from Latin 'castellum,' a diminutive of 'castrum,' a fortified place or camp — so a 'castle' is literally a 'little fort.' Stitch them together and you get a 'little fort of sand,' a phrase the seaside made inevitable.