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.
Words nearby leave flat
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.