不慎 (bù shèn) is a formal adjective meaning 'careless, incautious', but it is most often used adverbially before verbs to mean 'accidentally, inadvertently' (e.g., 不慎摔倒 'accidentally fell'). It is more formal and less common in everyday speech than 粗心 (cū xīn, HSK4), which describes a general careless attitude or habit. Unlike 草率 (cǎo shuài, HSK7, 'rash, sloppy') which implies haste, 不慎 focuses on unintended mistakes due to lack of caution.