(rare, obsolete) Used to represent /f/ or any voiceless fricative in the transliteration of foreign words.
This use is recommended in the hangulization standard that was in effect in South Korea from 1948 to 1958, and also in the International Korean Phonetic Alphabet developed by Lee Hyun Bok. While neither of these systems has been widely accepted, this letter is sometimes used in online writing for the same purpose.
ㆄ - Middle Korean
ㆄ (transliteration needed)
(not used after 1490) The voiceless bilabial fricative, /ɸ/. Used solely in representing the pronunciation of Chinese characters.