The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
adverb & adj. Music Abbr. rf
With a sudden increase of emphasis. Used chiefly as a direction.
Origin: Italian, present participle of rinforzare, to reinforce, strengthen : ri-, again (from Latin re-; see re-) + inforzare, to enforce, make strong (from Old French enforcier; see enforce).