From Longman Dictionary of Contemporary Englishopen your mouthopen your mouthSPEAK A LANGUAGEto prepare to speak ‘I’ll go, ’ Travis said quickly before she could open her mouth.open your mouth to say/speak/protest etc Julia opened her mouth to reply, but they were interrupted. → mouth
Examples from the Corpus
open your mouth• He stared, he threw back his head in glad greeting and opened his mouth.• She stumbled to her feet, clutching the eiderdown around her, and opened her mouth to call to them.• If you have to be sure before you dare open your mouth, most people are going to stay quiet.• He would croak if he opened his mouth, so he kept it shut.• I opened my mouth to shout for help, but nothing came out.• Angrily she opened her mouth to protest but at that moment the door opened and David appeared.• The baby robins, scrub jays, finches, sparrows and starlings opened their mouths wide in anticipation.• Even I wouldn't realise until they opened their mouths - which was only to sing the malai national anthem.• If you can get people to relax, they're more likely to open their mouths and talk. 