To open one's mouth - definition of To open one's mouth by The Free Dictionary
To open one's mouth
See also: Open
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive
There is also a mouth-widening tyre jack which helps to open one's mouth
wider in order to secure a flying sandwich.
Apparently they well understood the saying that it is better to keep silent and be thought stupid than to open one's mouth
and remove all doubt.