A level surface, without elevation, relief, or prominences; an extended plain; specifically, in the United States, a level tract along the along the banks of a river; as, the Mohawk Flats.(noun)
source: webster1913
adverb (r)
with flat sails(adv.all)
Example: Sail flat against the wind.
source: wordnet30
in a forthright manner; candidly or frankly(adv.all)
Example: Came out flat for less work and more pay.