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Webster 1828 Edition

Adj.

[from flat.] Somewhat flat; approaching to flatness.

flattish ‎(not comparable)

Somewhat flat, in any comparable sense of the word.
- 1938, George Orwell, Homage to Catalonia, Chapter 2,
  The hills in that part of Spain are of a queer formation, horseshoe-shaped with flattish tops and very steep sides running down into immense ravines.
- 2011, David Taylor, The Complete Contented Cat: Your Ultimate Guide to Feline Fulfilment
  The cobby cat is a solidly built animal with short, thick legs, broad shoulders and rump, and a short, rounded head with a flattish face.