Almost certainly a suffixed derivative of the Old Korean ancestor of Middle and Modern Korean 첫 (Yale: ches) "first".
初叱音 (*CHEsem)
This word is attested once in a twelfth-century gloss of the Avatamsaka Sutra.
In Old Korean orthography, native terms with clear Chinese equivalents are usually written with an initial Chinese character glossing the meaning of the word, followed by one or more Chinese characters that transcribe the final syllable or coda consonant of the term. In the case of 初叱音, the first character shows that this is the native Old Korean word for "first", and the subsequent characters show that the final syllable of this word involves *-s and *-m. Because the semantics and the final syllable match, the word is conventionally reconstructed as *CHEsem, the ancestor of 처〮ᅀᅥᆷ (Yale: chézèm, “the first time”). Besides the orthographic evidence (see 叱#Phonogram), there is growing scholarly consensus that Middle Korean /z/ arises from lenition of intervocalic Old Korean */s/.