Hangul syllable Meub

It is one Hangul syllable, built from an opening consonant, a vowel, and a final consonant.

In 믑, those parts are opening ㅁ, vowel ㅡ, and final ㅂ. The block represents one syllable; its place in the Korean word supplies the word-level meaning.

Hangul was devised under King Sejong, a Korean ruler, to write Korean speech more directly than Chinese-character-based systems. The royal chronicle Sejong Sillok records the new script in 1443; an explanatory work called Hunminjeongeum (“Correct Sounds for the Instruction of the People”) was published in 1446. It joined a writing landscape that also used Hanja (Chinese characters in Korean), Idu (Chinese characters adapted to write Korean), and kugyeol (marks that guided reading of Chinese Buddhist texts); Hangul later spread beyond elite writing while Chinese characters remained in official and scholarly use for a time.

Someone sent me 믑—what should I make of it?

Read it as Korean text; the surrounding message supplies the clue.

By itself, the character does not tell you whether it is a fragment or an intentional one-syllable message, and it does not settle the sender’s tone. Ask what word or phrase it belongs to.