Hangul Syllable Sih

It is a Hangul syllable block, not a complete word.

Hangul is the Korean alphabet; its letters, called jamo, form syllable blocks. Here ㅅ begins the block, ㅣ is the vowel, and ㅎ closes it. Fonts can vary the spacing and proportions inside the block without changing which syllable it is.

King Sejong created Hangul as Hunminjeongeum in 1443 to make Korean speech easier to write than through the Chinese-character methods then in use. A book explaining its design appeared in 1446. Hangul became Korea's official national writing system in 1894; those dates describe the alphabet, not a special origin for 싷.

What should I make of 싷 sent by itself?

Treat it as a fragment and ask what the sender meant.

Without a surrounding Korean word or sentence, one syllable does not tell you the intended message. To tell 싷 or 싫 apart, check the final letters: 싫 appears in 싫다 (“to dislike”) and has ㄹㅎ; 싷 has ㅎ alone.

How do I type 싷 in Korean?

Use a Korean input method and enter ㅅ, ㅣ, then ㅎ.

A Korean IME composes those pieces into the syllable block. If the field leaves them separate, switch to an IME that combines Hangul letters into syllables.