率尔 (shuài ěr) is an adverb meaning 'casually' or 'offhandedly', describing actions done without careful thought, preparation, or deliberation. The character 率 (shuài) originally means 'to lead' but in classical usage conveys spontaneity or rashness; 尔 (ěr) is a classical particle that softens tone and adds a sense of immediacy or manner—like 'thus' or 'in this way'. Together, they evoke a breezy, unreflective attitude, often with a subtle hint of recklessness or informality.

This term is primarily used in written Chinese and formal speech—not everyday conversation—and carries a slightly literary or classical flavor. It frequently appears in essays, historical narratives, or critiques to describe hasty decisions, impulsive remarks, or unplanned behavior. While not inherently negative, it often implies a lack of due diligence, so context matters: saying someone answered 率尔 may suggest their reply was too quick to be reliable, whereas 率尔应允 suggests agreeable but perhaps unconsidered consent.