Definition & Meaning of "run off at the mouth"

What is the origin of the idiom "run off at the mouth" and when to use it?

The idiom 'run off at the mouth' originated in the early 1900s and is used to describe someone who talks excessively and continuously without thinking. It suggests that the person speaks without restraint or control, like words are running out of their mouth uncontrollably. The phrase captures the irritation often felt by listeners towards those who seem unable to stop talking, going on and on without pause. The idiom is used informally and usually with a negative connotation to criticize someone for talking too much without purpose.