1
general::
1. In a direct manner, bluntly. For example, He told the true story flat out. [Colloquial; mid-1900s] 2. At top speed, as in She was running flat out to catch the train. [Slang; c. 1930]
American Heritage Idioms
2
general::
Phrase(s): flat out
1. clearly and definitely; holding nothing back. • I told her flat out that I didn’t like her. • They reported flat out that the operation was a failure. 2. at top speed. • How fast will this car go flat out? • This car will hit about 110 miles per hour flat out.
McGrawhill's American Idioms And Phrasal Verbs