flattest

Having no variations in height.

“The land around here is flat.”

(voice) Without variations in pitch.

Having small or invisible breasts and/or buttocks.

“That girl is completely flat on both sides.”

(note) Lowered by one semitone.

Of a note or voice, lower in pitch than it should be.

“Your A string is too flat.”

(of a tire or other inflated object) Deflated, especially because of a puncture.

Uninteresting.

“The party was a bit flat.”

(of a carbonated drink) With all or most of its carbon dioxide having come out of solution so that the drink no longer fizzes or contains any bubbles.

Lacking acidity without being sweet.

(of a battery) Unable to emit power; dead.

(of a throw) Without spin; spinless.

Lacking liveliness or action; depressed; dull and boring.

“The dialogue in your screenplay is flat -- you need to make it more exciting.”

Absolute; downright; peremptory.

“His claim was in flat contradiction to experimental results.”

(of a consonant) sonant; vocal, as distinguished from a sharp (non-sonant) consonant

(grammar) Not having an inflectional ending or sign, such as a noun used as an adjective, or an adjective as an adverb, without the addition of a formative suffix; or an infinitive without the sign "to".

“Many flat adverbs, as in 'run fast', 'buy cheap', etc. are from Old English.”

(of a golf club) Having a head at a very obtuse angle to the shaft.

(of certain fruits) Flattening at the ends.

(authorship, especially of a character) Lacking in depth, substance, or believability; underdeveloped; one-dimensional.

“The author added a chapter to flesh out the books' flatter characters.”

Fixed; unvarying.

“a flat fare on public transport”