from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A castlelike structure built of wet sand, as by children at a beach.
- n. Something that lacks substance or significance.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A sculpture made of sand and resembling a miniature castle; typically, but not always, made for fun by a child on a beach.
sand + castle (Wiktionary)