걱정 마, 나 입 하나는 무거워

The line, explained

In every Korean crime film there is a moment in a back room or a parked car. Someone has seen too much, and someone else is deciding whether to trust them. The nervous one leans forward and swears: don't worry, my mouth is heavy. Korean pictures secrecy as weight; a heavy mouth does not open easily, while a light one flaps in the wind. Today's line is that promise, and the little word 하나는 makes it sound like the one virtue the speaker is sure of.

걱정 마 is 걱정, "worry", plus 마, the casual "don't", from the verb 말다. It is a shortened 걱정하지 마, and both are everyday reassurance. 나 is "I", with no particle, as is normal in speech. 입 is "mouth". 하나 means "one", and with the topic marker 는, N 하나는 means "this one thing, if nothing else". So 입 하나는 is "my mouth, at least", implying "I may have other flaws, but this I can promise you".

무거워 is 무겁다, "to be heavy", in the casual present. It is an ㅂ-irregular adjective: before a vowel ending, the ㅂ turns into 우, so 무겁 plus 어 becomes 무거워. Its opposite works the same way: 가볍다, "light", becomes 가벼워. In fast speech, 입 and 하나 run together and the ㅂ meets the ㅎ to make a breathy ㅍ, so 입 하나는 sounds like i-pa-na-neun. Say it quickly, as one word, the way a nervous informant would.

The idiom normally takes the subject marker, 입이 무겁다, and works like any adjective: 입이 무거운 사람, "a discreet person", 입이 무거워서 믿을 만해, "she keeps secrets, so she's trustworthy". The opposite, 입이 가볍다, is a real insult, and so is the slangier 입이 싸다, "a cheap mouth". If you want someone to stay quiet, 입 다물어, "shut your mouth", is blunt; 비밀 지켜 줘, "keep it secret for me", is kinder and more common between friends.

Swap the opening and keep the promise: 믿어 봐, 나 입 하나는 무거워 (trust me), 안심해 (relax), 말해 봐 (go on, tell me), 걱정하지 마 (the fuller "don't worry"). Then try 하나는 on your own strengths: 나 노래 하나는 잘해, "if nothing else, I can sing", or 우리 엄마 요리 하나는 최고야, "whatever else, my mom's cooking is the best". It is a humble brag built from a single number and a particle.