FLAT (6)

Flat, a. 1. 1. (Golf) Having a head at a very obtuse angle to the shaft; — said of a club.
2. 2. (Gram.) 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 -ë, the loss of this ending having made them like the adjectives. Some having forms in ly, such as exceeding, wonderful, true, are now archaic.
3. 3. (Hort.) Flattening at the ends; — said of certain fruits.