What 뭘 means
It means “what” as an object.
뭘 먹어요? means “What are you eating?” The question word stays in its answer slot; English moves “what” to the front.
뭐 goes back to the 15th century. By the late 18th century it was used chiefly in questions; by the late 19th century it could also mean an unspecified thing. That is 뭐’s history, not 뭘’s origin.
Is 뭘 formal? No: it is neutral conversation; use fuller 무엇을 in formal writing or service language.
Don't confuse it with
1Combination ideas
1What are you eating?
Questions people ask
1Why can 뭘 have more than one Unicode form?
Unicode represents it as one Hangul block or separate Korean letters; text normalization makes both forms equivalent.
How to type 뭘
Windows (Word)
Alt+XType BB58, then Alt+X to get 뭘.
Windows
Win+Rrun charmap for the full table, or use the emoji picker's Ω symbols tab (Win + .).
macOS
Control+Command+SpaceOpen the character viewer and search “hangul syllable mweol”.
Linux
Ctrl+Shift+UType bb58, then Enter or Space.
HTML
뭘CSS
\BB58Codes and encoding1 code points
HTML decimal
뭘
HTML hex
뭘
CSS content
content: "\BB58";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%EB%AD%98
1
On screen
1
Code points
1
JS .length
3
UTF-8 bytes
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.