FLAT (32)

(a.) Not having an inflectional ending or sign, 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, are from AS. adverbs in -e, the loss of this ending having made them like the adjectives. Some having forms in ly, such as exceeding, wonderful, true, are now archaic.