Unlike most adverbs in English, flat adverbs (also known as plain adverbs) do not end in -ly.
Examples and Observations:
- "Drive slow and enjoy the scenery. Drive fast and join the scenery."
- "If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting."
(Jack Handy, Deepest Thoughts: So Deep They Squeak, Hyperion, 1994)
- "The Flat Adverb. In the early part of our period the Adverb with the Adjective was largely used without the termination -ly, as: 'exceeding glad' (Daniel vi. 23); 'exceeding sorry' (Mark vi. 26); 'grievous sick' ('Rich. II.' I.iv); 'a wonderful sweet air' ('Cymbeline,' II.iii); 'marvelous great' (Ps. xxxi. 23).
"A good text for observing the Flat Adverb is Robinson Crusoe: 'the weather being excessive hot'; 'extreme hot'; 'the sea went dreadful high.' This Flat Adverb is now archaic, and we rarely make new instances; but we retain many old ones, as when we say pitch dark, mighty fine."
(John Earle, A Simple Grammar of English Now in Use. G.P. Putnam's, 1898)
- "Near is called a 'flat adverb,' with the ly clipped off and morphing into the same form as its related adjective. 'Drive slow, think different, do right, hang tough.' Don't let this dual use get you down; the flat adverb is one of English's little confusions, and it sure (or surely--pick one) doesn't worry usagists."
(William Safire, "Don't Call Me, 'Near Elderly.'" No Uncertain Terms: More Writing From the Popular "On Language" Column. Simon and Schuster, 2003)
- "In EMnE [Early Modern English], plain adverbs (those identical in form to adjectives) were widely used, even by careful writers. The list of acceptable plain adverbs today has shrunk to a few frequent ones, which often seem to have survived only because the corresponding form in -ly has a different meaning. We say 'I worked hard until very late' because hardly and lately do not mean the same thing as hard and late. Except for a handful of common time words like early, daily, weekly, and hourly, even adjectives that already end in -ly are at best uncomfortable when used adverbially. For example, though contemporary dictionaries still list friendly as an adverb, most of us would hesitate to write it as such, preferring a paraphrase like in a friendly manner or even the phonological monstrosity friendlily (also recognized by some dictionaries).
"Some of the common closed-list adverbs (those not derived from adjectives) of EMnE have since become obsolete or at least archaic. Examples include afore 'before,' ere long, without 'outside, out of doors.' hither, and thither."
(C.M. Millward, A Biography of the English Language, 2nd ed. Harcourt Brace, 1996)