嘴上没毛，说话不牢zuǐ shang méi máo , shuō hua bù láo

be young and talk too much; downy lips make thoughtless slips;/老曹说我‘～’，不要扰乱了人心……(柳青《铜墙铁壁》43) …he said I was young and talked too much,that I shouldn’t get the people worked up over nothing. …/俗话说： ‘～。’ 再过上五七个年也许差不多。(柳青《铜墙铁壁》21) As the old saying goes: ‘Downy lips make thoughtless slips.’ Maybe in five or six years he’ll be ready for it.