fluff (one's) lines
To forget or bungle the words that one is supposed to say, as in a speech or play. My mind went completely blank as soon as I stepped on stage, and I totally fluffed my lines. I had learned the speech off by heart, but I was so emotional that I started fluffing my lines about halfway through.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
fluff one's linesand blow one's lines; muff one's lines
to speak one's speech badly or forget one's lines when one is in a play. The actress fluffed her lines badly in the last act. I was in a play once, and I muffed my lines over and over. It's okay to blow your lines in rehearsal.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.