pipe dream
US /ˈpaɪp ˌdrim/
UK /ˈpaɪp ˌdrim/

1.
白日梦, 痴心妄想
an unrealistic hope or fantasy
:
•
His plan to become a millionaire overnight is just a pipe dream.
他一夜暴富的计划只是个白日梦。
•
Getting a promotion without working hard is a pipe dream.
不努力工作就想升职是痴心妄想。