拦路虎 (lánlùhǔ): stumbling block

Literally 'a tiger blocking the road' — 拦 (block) + 路 (road) + 虎 (tiger). Pre-modern travelers feared man-eating tigers as the worst hazard on mountain paths, so the figurative leap to 'biggest obstacle in your way' is intuitive. The flavor in modern Chinese is purely metaphorical, equivalent to English 'roadblock' or 'stumbling block'.

history

In Yuan and Ming-era fiction, 拦路虎 (and the cousin term 拦路鬼) literally meant a highway bandit who would rob travelers on remote roads. As bandits faded, the metaphor took over, and today no Chinese speaker thinks of armed robbers when using this word.