Fiddle-faddle is a whimsical term that stands for foolish or inconsequential talk. If you are looking for antonyms to describe someone who is serious, thoughtful, or pragmatic, you might use words such as sincere, genuine, authentic, or earnest. On the other hand, if you want to contrast fiddle-faddle with someone who is sharp, witty, or incisive, you might use adjectives like astute, shrewd, perspicacious, or insightful. Similarly, if you want to substitute fiddle-faddle with a more constructive conversation, you might use words like sensible, practical, logical, or substantive. Ultimately, the antonyms for fiddle-faddle will depend on the context of the conversation and the tone you want to convey.