稀世 is almost always attributive and pairs with a small set of nouns: 稀世珍宝 (rare treasure), 稀世奇才 (extraordinary talent), 稀世之宝 (treasure rare in the world). Don't use it predicatively — say 非常罕见 (very rare) instead.
memory
稀 'rare/sparse' + 世 'world' — 'rare in [all the] world.' Same 稀 as in 稀有 (xīyǒu, rare/scarce).
