입이 무겁다

This phrase is deeply rooted in ancient Confucian values that dominated the Joseon Dynasty. Back then, 'lightness' of speech was seen as a serious character flaw for a gentleman or scholar. A 'light' person (입이 가볍다) was considered frivolous, untrustworthy, and lacked self-discipline. Silence was often equated with wisdom and internal strength. In traditional Korean folk wisdom, words were seen as things that, once released, could never be gathered back—like spilled water. To prevent this 'spillage,' one needed a 'heavy' mouth to act as a sturdy dam. It wasn't just about hiding bad things; it was about the dignity of choosing when and how to speak. Even today, being called 'heavy' in this context is a badge of honor, whereas in English, 'heavy' usually just means you need to hit the gym!

Using this phrase is quite simple because it functions like a standard adjective. The base form is 무겁다 (to be heavy). In a polite, everyday conversation, you would say 입이 무거워요. If you are talking about a specific person, you can use it as a modifier: 입이 무거운 사람 (a person with a heavy mouth). You’ll often hear it paired with adverbs like 정말 (really) or 워낙 (by nature). For example, 'Minho is naturally discreet' becomes 민호는 워낙 입이 무거워요. You can also use it to ask for a favor. If you're about to spill some tea and want to make sure your friend won't tell, you might ask, 'Is your mouth heavy?' (너 입 무거워?). Just don't say it while they are actually eating a heavy burger, or they might get confused!

Imagine you're in a company Slack channel and you accidentally find out who is getting the big promotion. You whisper it to your work bestie and add, 'Please, keep this quiet. You know I’m telling you because you have a heavy mouth.' This is a classic professional scenario. Or consider a K-drama plot where the protagonist confesses their crush to a childhood friend. The friend might say, 'Don't worry, my mouth is heavy,' which is basically Korean for 'I'm taking this secret to my grave.' Even in digital spaces, like a group chat on KakaoTalk, if someone is known for not leaking chat logs or screenshots, people will say they have a heavy mouth. It's the ultimate 'Safe Space' certification.

This is your go-to phrase when you want to praise someone's integrity. Use it during job interviews when describing your strengths—employers love a 'heavy mouth' because it means you won't leak company secrets to competitors. Use it when introducing a friend to someone else to show they are trustworthy: 'You can tell her anything; she’s very discreet.' It’s also great for social media captions when you’re posting a photo with a long-time friend who has kept your secrets since middle school. It shows a deep level of respect and appreciation for their loyalty. It’s a warm, affirming thing to say to someone you truly value.

Be careful: 입이 무겁다 is NOT for people who are just shy or don't talk much. If someone is socially anxious and stays quiet at a party, calling them 'heavy-mouthed' would be weird. For that, you should use 말수가 적다 (to have few words). Also, don't use it to describe someone who is physically unable to speak or someone who is being stubborn and refusing to answer a direct question in an argument. That’s more like 'closing one's mouth' (입을 다물다). Finally, avoid using it for someone who is keeping a *bad* secret, like a crime or a lie that hurts others. In those cases, the 'weight' of the mouth feels more like a burden than a virtue.

Learners often try to translate 'sealed lips' or 'zippered mouth' literally into Korean. While 입에 지퍼를 채우다 (to zip one's mouth) exists as a newer expression, 입이 무겁다 is much more common and natural. Another mistake is using the word for 'closed' (닫히다) instead of 'heavy.' ✗ 입이 닫혔어요 sounds like your jaw is locked or you have a medical issue! Always stick with the weight metaphor. Also, remember the subject marker 이. Some people say 입을 무겁다, but since 'heavy' is a descriptive verb (adjective) in Korean, it needs the subject marker, not the object marker. If you get it wrong, don't worry—your Korean friends' mouths are heavy, so they won't tell anyone about your grammar fail!

If you want to mix it up, you can use 입이 가볍다 (mouth is light) as the direct opposite for a gossip. To say 'keep a secret' more formally, you'd use 비밀을 지키다. A more idiomatic version of 'keeping a secret' is 입을 단단히 단속하다, which literally means to 'tightly supervise/crack down on one's mouth.' It sounds a bit like your mouth is a high-security prison. There is also 입술을 깨물다 (to bite one's lips), but that usually implies holding back emotions or pain rather than keeping secrets. Stick to the 'heavy mouth' for maximum 'native speaker' points.

Is this phrase formal? It’s neutral! You can use it with your boss or your younger brother just by changing the ending (무거우십니다 vs 무거워). Can it be used for kids? Usually, no. It implies a level of adult responsibility and character that we don't typically expect from a five-year-old who will tell the whole playground you still sleep with a teddy bear. Does it mean someone is slow at speaking? Not at all. A fast talker can still have a 'heavy mouth' if they are careful about *what* they say. Trust the weight, and you'll master this idiom in no time!