Hangul syllable Euss

No; 읐 is a Hangul block, not a complete Korean word.

Hangul is the writing system used for Korean. 읐 combines initial ㅇ, vowel ㅡ, and final consonant ㅆ. Those parts show how the block is built; they do not translate it into a word. In Korean text, read the whole word around it.

Hangul was developed in 1443 and promulgated in 1446 as a writing system for Korean. It offered a more accessible way to write Korean than adapting Chinese characters. 읐 belongs to Hangul’s regular syllable system; its history is part of that system, not a separate origin story for this block.

How do I type 읐?

With a Korean IME, enter ㅇ, then ㅡ, then ㅆ.

A Korean input method composes that initial-vowel-final sequence into the syllable block. Entering separate jamo directly may leave them as separate characters.

The precomposed 읐 and the decomposed sequence 읐 are different stored forms, even when a font displays both as one block. Use the precomposed form when the stored text must contain this single character.

Should I read a lone 읐 as a reply?

No; a lone 읐 needs context before you translate it.

If someone sends only 읐, ask what they meant or where it came from. In a Korean word, read it as one syllable and translate the whole word; alone, it does not safely mean agreement, laughter, surprise, or refusal.