open one's big mouth UK /ˈəʊ̯.pən ˌwʌnz bɪɡ‿maʊ̯θ/US /ˈoʊ̯.pən ˌwʌnz bɪɡ‿maʊ̯θ/ verb1 •(informal) To speak about things rashly or foolishly, when it would be better to stay silent.Grr, why do I always open my big mouth after doing something wrong? formsopened one's big mouth · opening one's big mouth · opens one's big mouth