The word "挑肥拣瘦" is formed by juxtaposing two verb-object phrases: "挑肥" (pick the fat) and "拣瘦" (choose the lean), where the characters "挑" and "拣" are synonymous verbs meaning to select, and "肥" and "瘦" are antonymous adjectives meaning fat and lean. This parallel structure combines opposite objects with similar actions to create a figurative meaning, conveying the idea of being overly selective or nitpicking.