1
general::
Phrase(s): leave someone flat
1. Fig. to fail to entertain or stimulate someone. • Your joke left me flat. • We listened carefully to his lecture, but it left us flat. 2. Fig. to leave someone without any money—flat broke. • Paying all my bills left me flat. • The robber took all my money and left me flat. 3. Fig. to leave someone completely and suddently alone. • I was at the dance with Harry, but when he met Alice, he left me flat. • They just walked off and left us flat.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Forsake or abandon completely, especially without warning. For example, He didn't tell her he wasn't picking her up; he just left her flat. [Colloquial; c. 1900] Also see HIGH AND DRY; LEAVE IN THE LURCH.
American Heritage Idioms