How did the compound 莫 + 非 semantically shift to mean "can it be/is it possible + that"? ABC Chinese–English Comprehensive Dictionary (2003), p 628. I can't access the &quot;slightly revised through July 2005&quot; ed. 莫非 - Wiktionary
appears correct.


Am I correct that 莫 here means 3 below?



none; nothing; not anything

(literary or dialectal Mandarin, dialectal Cantonese, Gan, Hakka, Min Dong) do not; don't; must not

particle indicating a rhetorical question

A surname​.




I don't know which meaning fits 非?



not be; is not; not
(logic) NOT
wrong; incorrect
to reproach; to blame
(colloquial) Used to insist on something. to have got to; to simply must



Still I don't understand how these two characters compound to signify &quot;can it be that&quot;?  We're combining merely two characters that signify negation.

Don't we need more characters to signify the modal verb 'can', eg &quot;possibility in either a dynamic, deontic, or epistemic sense&quot;? Something feels missing!
Both 莫 and 非 denote the sense of not.  The combination 莫非(a double negative) is a set phrase to mean 难道. It's often used to initiate a rhetorical question.
E. g.  莫非你不知道？ / 难道你不知道? // Don't you know it?