信口

信口	xìn kǒu	to blurt sth out / to open one's mouth without thinking
信	xìn	letter / mail / CL:封 / to trust / to believe / to profess faith in / truthful / confidence / trust / at will / at random
口	kǒu	mouth / classifier for things with mouths (people, domestic animals, cannons, wells etc) / classifier for bites or mouthfuls