Potentially a compound of *mwo (some sort of root) + *to (semantically light noun meaning "objective fact", whence Middle Korean ᄃᆞ (to)) + *-l/r (some kind of suffix). Compare Old Korean 不冬 (*ANtol).
毛冬 (*mwotol)
This form is used in the orthography of the hyangga poems, written between the late seventh and late tenth centuries, and once in a 1262 Idu script document.
The Interpretive Gugyeol glosses to the Buddhist canon written between the tenth and thirteenth centuries, which represent the largest and best-understood corpus of the Old Korean language, instead use two different forms to denote inability. The "Yogācārabhūmi-Śāstra glossing tradition", which appears to have been more mainstream, consistently uses the adverb 不只 (conventionally *ANTOk). The divergent "Avatamsaka Sutra glossing tradition" typically uses the adverb 不㇒, which for a long time remained uninterpretable. The recent discovery (in 2009) and interpretation (in the late 2010s) of a gloss of the Sutra of the Repentance Ritual of Great Compassion, which belongs to the latter tradition, gives suggestive evidence that ㇒ was a graphic abbreviation involving the first stroke of 毛, and that 不㇒ was to be read as *mwotol. For a similar example of a gugyeol gloss which was read out as to replace the actual Chinese equivalent, see 氵 (*sam-).
The reason that the two glossing traditions used apparently different words to express "cannot", and whether 不只 and 毛冬 are related or unrelated terms, remains unknown.
As with Middle and Modern Korean (see 못 (mot) and 못하다 (mothada)), the Old Korean 毛冬 construction had two forms: a short form in which the adverb directly preceded the negated verb, and a long form in which the adverb negated the verb 爲 (*ho-, “to do”) similar to English do-support. However, unlike in Middle and Modern Korean, 爲 did not act as a true auxiliary verb, as the main verb was nominalized to become the direct object of 爲.
According to the analysis of Mun Hyeon-su, when the main verb was modified by another adverb, word order in the long form construction of 毛冬 differed from the long form construction of 不只: