Translation guide
Describes someone who talks too much, reveals secrets, or boasts loudly. Japanese uses different expressions depending on whether the person is a gossip, a braggart, or just loud.
To describe someone who reveals confidential information or talks carelessly about private matters.
Literally 'light-mouthed'. The most common and natural way to say someone has a loose tongue and easily spills secrets.
彼は口が軽いから、秘密を話さないほうがいい。
He has a big mouth, so you'd better not tell him secrets.
Literally 'mouth slips'. Used when someone accidentally lets something slip, rather than habitually blabbing.
つい口が滑って、サプライズのことを言ってしまった。
I accidentally let it slip and told him about the surprise.
A noun or na-adjective meaning 'chatterbox' or 'talkative person'. Can imply indiscretion, but also just someone who talks a lot.
彼女はおしゃべりだから、何でもすぐ広まっちゃう。
She's such a big mouth; everything spreads quickly.
Literally 'tight-mouthed'. The opposite of 'big mouth'. Included here because learners often encounter it in contrast. Use with negation to mean 'not tight-lipped'.
彼は口が堅くないから、信用できない。
He doesn't have a tight mouth, so I can't trust him.
To describe someone who brags about their abilities, achievements, or possessions in an annoying way.
Literally 'to beat a big mouth'. Means to talk big, boast, or make exaggerated claims. Often used for empty bragging.
彼はいつも大口を叩くけど、実際は何もできない。
He always talks big, but in reality he can't do anything.
Similar to 大口を叩く, but slightly more colloquial. Means to speak arrogantly or presumptuously.
新入りのくせに大きな口をきくな。
Don't talk so big when you're just a newcomer.
A 'braggart' or 'blowhard'. Someone who tells tall tales or exaggerates their exploits.
To describe someone whose voice is loud or who dominates conversations without necessarily revealing secrets or boasting.
Literally 'voice is big'. The most straightforward way to say someone has a loud voice. Can imply they are a big mouth in the sense of being loud.
彼は声が大きいから、静かな場所では目立つ。
He has a big mouth, so he stands out in quiet places.
As above, can simply mean a talkative person, not necessarily indiscreet.
うちの母はおしゃべりで、電話が長いんだ。
My mom is a big mouth and stays on the phone for ages.
口が軽い is about leaking secrets; 大口を叩く is about arrogant boasting. Don't confuse them. If you say 口が軽い when someone is bragging, it sounds like they are spilling secrets, which is odd.
彼は口が軽い（＝秘密を漏らす）が、大口は叩かない。
He has a loose tongue (leaks secrets) but doesn't talk big.
The direct translation '大きい口' (big mouth) is not used to describe a person's character in Japanese. It only refers to a physically large mouth. Always use idiomatic expressions.
That guy is a big mouth, so you should take what he says with a grain of salt.