The phrase "in nothing flat" is an idiomatic expression that means very quickly or promptly. Its antonyms are words or phrases that convey the opposite meaning, such as slowly, gradually, or bit by bit. Other antonyms could include sluggishly, leisurely, or in a measured way. When someone moves at a slow pace or takes their time, it can be said that their actions are the exact opposite of being done in "nothing flat." Using antonyms can help to convey a completely different tone, mood, or atmosphere, and can be useful for writers looking to create contrasting effects in their works.