How could Joseph Smith correctly give the meaning of “Shinehah” in Abraham 3:13 (a word that applied during a narrow span of about 6 centuries comprising the likely time of Abraham’s life)?

In Abraham 3:13 we read:

And he said unto me: This is Shinehah, which is the sun. And he said unto me: Kokob, which is star. And he said unto me: Olea, which is the moon. And he said unto me: Kokaubeam, which signifies stars, or all the great lights, which were in the firmament of heaven.

Shinehah can indeed mean the sun in ancient Egyptian, as explained by the Book of Mormon Onomasticon:

As Egyptian, SHINEHAH could certainly be the “sun,” since it apparently incorporates Egyptian šn(w), the name for the solar “circuit,” and for the old “cartouche” – “emblem of sun’s orbit, which symbolizes eternity”; + Egyptian nḥḥ “eternity, forever” (with solar determinative ⊙) > Coptic eneḥ “eternity,” šaeneḥ “forever”; or Egyptian ḥḥ > Coptic ḥaḥ “million, large number,” which may be related.
– Book of Mormon Onomasticon – Shinehah

Where did Joseph Smith get the word “Shinehah” from? How did he know it means the sun? Why is this word only attested during a span of about six centuries overlapping with the likely time that Abraham lived?

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