Something is flat when it is
lacking in the characteristics that make it
useful/
interesting/
unique. As noted above, this applies to soda without
carbonation, women with small
tits, and
Kansas, but also to characters in
literature or
movies with little
personality,
tires without
air, and
small animals that have been run over.
As a side note, the only soda that tastes good after it is flat is Dr. Pepper, or any one of the Dr. Pepper imitations.