A 之又 A（少之又少）
extremely A; A in the extreme (literary intensifier)
Structure
单音节形容词 A ＋ 之又 ＋ A ： 少之又少 ／ 慎之又慎 ／ 难之又难
What it means
A literary intensifier: a one-syllable adjective doubled around 之又 ('and again'), so 少之又少 is 'few and fewer still', extremely rare. Modelled on the Daodejing's 玄之又玄.
How it is built
One-syllable adjective A + 之又 + A, as a predicate or after 要／必须 (必须慎之又慎): 这样的化石少之又少 (Fossils like this are extremely rare).
When to use it
Only a handful are in common use: 少之又少 and 慎之又慎 above all, then 难之又难 and 精之又精. Written and formal; in conversation say 特别少 or 非常小心.
Watch out
Not with two-syllable adjectives (✗稀少之又稀少), and no 很 in front (✗很少之又少). Stick to the common set rather than coining new ones.
Examples
能把羽毛也保存下来的恐龙化石，在世界上少之又少。
Dinosaur fossils that preserve the feathers as well are extremely rare anywhere in the world.
修复文物必须慎之又慎，一个小错误就可能造成无法挽回的损失。
Restoring cultural relics calls for the utmost care: one small slip can do irreparable damage.
要在几十万块碎骨里找到这一小块，真是难之又难。
Finding this one small piece among hundreds of thousands of bone fragments was hard in the extreme.
Learn this in Lesson 165 · HSK 7–9: 辽西的带毛恐龙 (The feathered dinosaurs of Liaoning)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.
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