FLATNESS
languor, lethargy, sluggishness, phlegm, flatness
(noun) inactivity; showing an unusual lack of energy; “the general appearance of sluggishness alarmed his friends”
flatness, lusterlessness, lustrelessness, mat, matt, matte
(noun) the property of having little or no contrast; lacking highlights or gloss
flatness, two-dimensionality, planeness
(noun) the property of having two dimensions
flatness
(noun) a deficiency in flavor; “it needed lemon juice to sharpen the flatness of the dried lentils”
flatness
(noun) a want of animation or brilliance; “the almost self-conscious flatness of Hemingway’s style”
Source: WordNet® 3.1
Etymology
Noun
flatness (countable and uncountable, plural flatnesses)
The state of being flat
The state of being two-dimensional; planar : planarity
The state of being bland : dullness
Mattness, the quality of a painted surface which scatters or absorbs the light falling on it, so as to be substantially free from gloss or sheen
(oenology) Lack of bouquet and freshness of a wine, through too much aeration or infection with film yeasts
(statistics) slim-tailedness or platykurticity
Source: Wiktionary
Flat"ness, n.
1. The quality or state of being flat.
2. Eveness of surface; want of relief or prominence; the state of
being plane or level.
3. Want of vivacity or spirit; prostration; dejection; depression.
4. Want of variety or flavor; dullness; inspidity.
5. Depression of tone; the state of being below the true pitch; --
opposed to sharpness or acuteness.
Source: Webster’s Unabridged Dictionary 1913 Edition