Dream

nightmare dressed like a daydream

Meaning: Describes something that seems very pleasant or attractive at first but eventually reveals itself as horrible.

Example: Don't let her beauty fool you. She's just a nightmare dressed like a daydream. Read on

brown study

Meaning: melancholy mood accompanied by deep thought

Example: I realised that Sandra wasn't concentrating on what the boss was saying and had disappeared into a brown study. Read on

pipe dream

Meaning: an impossibility

Example: His plans of becoming an astronaut are a pipe dream, he should be more realistic. Read on

never in wildest dreams

Meaning: a situation beyond the scope of one's imagination

Example: Never in my wildest dreams did I think she'd abandon her father. Read on