Open any Korean comment section and half the sentences end in a bare ㅁ. Your textbook never mentions this, because textbooks teach -(으)ㅁ as a way to turn verbs into nouns and stop there.
Used at the end of a sentence, -음 turns the whole statement into a noun phrase, which means it carries no politeness level at all.
That last part is the entire point.
What it looks like
- 밥 먹었음 (bap meogeosseum) = ate
- 웃김 (utgim) = hilarious
- 인정함 (injeongham) = agreed, true
- 이거 맞음 = this is right
- 읽음 (ilgeum) = read, the label your phone shows when someone opens your message
Formation is simple. Add -음 after a consonant, -ㅁ after a vowel. 먹다 gives 먹음, 하다 gives 함, 웃기다 gives 웃김.
Why it exists
Every normal Korean sentence ending forces a decision. 먹었어요 is polite, 먹었어 is casual, and choosing between them means deciding your relationship to whoever is reading.
In a comment section you have no idea who that is. Could be a teenager, could be someone twice your age. And writing 먹었습니다 to strangers on a forum reads as stiff and faintly embarrassing, the way overly formal English does in a group chat.
-음 sidesteps the whole problem. There is no speech level attached, so there’s nothing to get wrong. Add that everyone else is already writing this way and it becomes the default rather than a choice anyone is making.
The honorific still works
This is the part I find genuinely elegant.
교수님 오늘 휴강하심 = the professor cancelled class today
하시다 plus ㅁ gives 하심. The honorific -시- survives, because -시- respects the subject of the sentence, while politeness endings respect the listener. Dropping the second doesn’t touch the first.
So you can be respectful about the professor while refusing to commit to a register with your reader. Korean lets you separate those two things, and this is one of the clearest places you see it.
Who writes this way
Not a young-people thing, and not a gaming-forum thing. People in their thirties and forties who grew up online write this way too.
It also isn’t confined to public posting. It shows up in KakaoTalk between friends, which surprised me when I first noticed it. If you were treating it as a specifically anonymous-internet register, that’s too narrow.
Where it doesn’t go
Anywhere with a named recipient who outranks you. Email, a message to a professor, anything at work.
The reason is the same as the reason it works online. Refusing to pick a speech level is fine when your reader is a crowd. Aimed at one specific person who is senior to you, that refusal reads as a choice, and the choice reads as cold.
Reading it vs writing it
Learn to read it immediately, because you’ll see it constantly and it strips off the verb endings you were trained to look for.
Writing it is fine in a comment section, and it’s genuinely one of the safer things a learner can imitate, since there’s no politeness level to get wrong. The mistake to avoid isn’t the form. It’s the setting.