irresistible force; overwhelming momentum
A set phrase likening someone's energy or momentum to a lion charging fiercely. Often used with verbs like 進む, 攻める, or 働く to describe unstoppable action.
He finished the work with irresistible force.
新入社員が獅子奮迅の勢いで営業先を回っている。
The new employee is visiting clients with overwhelming momentum.
破竹の勢い also means irresistible momentum, but comes from the image of splitting bamboo and is often used for a series of uninterrupted victories or successes. 獅子奮迅の勢い emphasizes fierce, lion-like energy.
From the Chinese idiom 獅子奮迅 (shīzi fènxùn), meaning 'a lion exerting itself fiercely'. The phrase entered Japanese and combined with の勢い to form a set expression describing unstoppable force.