Sigh: you've driven me tpo the dictionary, folks ....

riff Jazz ~n 1. an ostinato played over changing harmonies. ~vb. 2. to play or perform riffs. [C20: probably altered and shortened from REFRAIN.]

As it happens, I use the first computer-sorted Collins's (© 1979) with the interesting mistakes still present, so pinches of salt may be in order.