run off at the mouth

run off at the mouth{AmE}
run one's mouth (off) {BrE}
(to talk incessantly, babble, talk too much; talk when you should be quiet; to talk a lot without saying anything important ) — трещать без умолку ; слишком много говорить (ни о чем); ~ много молоть языком попусту; ≅ язык без костей

Example: I wish you would stop running off at the mouth. — Ты можешь немного помолчать, наконец?!

Example: He's just another one of these politicians who run off at the mouth. — Очередной политик, у которого язык без костей. (thefreedictionary.com)

Example: Tom runs off at the mouth too much. I wish he would temper his remarks. (thefreedictionary.com)

Example: Wilbur is always running off at the mouth about his investments. (answers.com)

Example: He was mean and angry and he just kept running his mouth. (thefreedictionary.com)

This idiom transfers a flow of water to an unending flow of words. (answers.com)

Someone who runs off at the mouth blathers, blurts, or babbles and is often called a motor mouth or a chatty Cathy (болтун, болтушка; трещотка, тараторка).

Related vocabulary:
talk [nineteen to the dozen]
talk someone's ear / head / arm /pants off
talk a blue streak
talk until one is blue in the face
talk the bark off a tree / the hind leg off a donkey (horse).
[not hear the end of sth]
[словесный понос]