bull-whip (n.)

also bullwhip, "long, thick type of whip 'used by drovers to intimidate refractory animals'" [Century Dictionary], 1852, American English, from bull (n.1) + whip (n.). Whips made of bull hide were known in Middle English as bull-rope, bull-sinew, bull-yerde. But in bull-whip (also in 19c. American English bull-whack) the sense is perhaps "whip that can drive a bull," or bull might be a reference to the size of it. The earliest references (1850s) are in Northern accounts of Southern slavery. As a verb from 1895.