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.
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.