Unlike my subject will I frame my song,
It shall be witty, and it shan’t be long.

The couplet is a self-referential promise about style: the speaker will not imitate the “subject” (implying something dull, prolix, or heavy), but will instead make the poem brisk and pointed. It plays on a familiar satiric posture—announcing, before the poem even begins, that the author values wit and brevity over solemn expansiveness. In Chesterfield’s literary persona (as in his better-known prose), polish, social intelligence, and economy of expression are treated as marks of good taste. The lines also function as a comic apology in advance: whatever follows will be intentionally short, and its chief aim is to entertain rather than to exhaust the reader.