Definition & Meaning of "knuckle up"in English

What does "to knuckle up" mean in street slang?

The structure combines "knuckle" (referring to the bony part of a fist) with "up" (indicating readiness or preparation), forming a verb phrase that describes preparing to fight. It is used to describe getting into a fighting stance, clenching fists, or showing readiness to engage in a physical confrontation, often as a response to a threat or challenge. People typically say "knuckle up" when daring someone to fight, such as "If you have a problem, knuckle up then."