Inherited from Old Spanish rabdo, from Latin rapidus (“swift”), via voicing of intervocalic /p/ to /b/, syncope of the following vowel, and vocalization of coda /b/ to /u̯/. Cf. the evolution of Latin cubitum > Old Spanish cobdo > modern codo, where the /u̯/ was absorbed into the preceding rounded vowel. Doublet of rápido, a borrowing from Latin.