A dream or an unrealistic, impractical plan; literally, a 'castle in the air'.

Origin: The word 'Luftschloss' is composed of two German words: 'Luft' meaning 'air' and 'Schloss' meaning 'castle'. It first appeared in the German language in the 19th century, possibly as a calque of the English phrase 'castle in the air'.
Historical and Cultural Context: In its earliest usage, 'Luftschloss' depicted an unattainable dream or a fanciful plan, often used in literature and philosophy to critique overly ambitious or unrealistic ideas.
Evolution: While the component words 'Luft' and 'Schloss' have remained unchanged, the concept of 'Luftschloss' has been used in German literature and poetry to illustrate the folly of grandiose but impractical schemes. Cognates in other languages include the English 'castle in the air' and the French 'château en Espagne'.
Modern Relevance: Today, 'Luftschloss' is used in German to describe fanciful or unrealistic ideas and remains a popular term in both casual conversation and formal writing, often to convey skepticism or critique.

The concept of a 'Luftschloss' is often humorously depicted in cartoons and illustrations as a literal castle floating in the sky, emphasizing the whimsical nature of such dreams.