What is the…
Sihor

also known as: Shi'hor and Shihor

—meaning: Waters of Horus

This name seems to refer to an ancient extremity of the Nile, possibly a canal, no longer in existence.

So David gathered all Israel together, from Shihor of Egypt even unto the entering of Hemath, to bring the ark of God from Kirjath-jearim. —1 Chronicles 13:5 KJV

And by great waters the seed of Sihor,
The harvest of the river, is her revenue;
And she is a mart of nations. —Isaiah 23:3 KJV / Compare Isaiah 23:3 ESV

And now what hast thou to do in the way of Egypt, to drink the waters of Sihor? … —Jeremiah 2:18 KJV excerpt

Biblical scholar and archaeologist Melvin Grove Kyle explains,

In the northeasternmost province of ancient Egypt, Khentabt (“Fronting on the East”), was a canal, a fresh-water stream drawn off from the Nile, called in the Egyptian language Shi-t-Hor, i.e. “the Horus Canal” (the -t- is an Egyptian feminine ending). There have been many changes in the branches and canals from the Nile in the Delta, and this one with many others has been lost altogether; but there is a tradition among the Bedouin of Wady el-`Arish to this day that once a branch of the Nile came over to that point.

This Shi-t-Hor, ‘Stream of Horus,’ makes perfectly clear and harmonious the different references of Scripture to South. It was ‘before Egypt,’ as Joshua describes it, and it was the first sweet water of Egypt which the traveler from Palestine in those days was able to obtain, as the words of Jeremiah indicate.

‘To drink the waters of South’ meant to reach the supply of the fresh water of the Nile at the border of the desert.

The two other references to South (1 Chronicles 13:5 KJV; Isaiah 23:3 KJV) are perfectly satisfied by this identification. The ‘seed of South’ (Isaiah 23:3 KJV) would be grain from Egypt by way of the Shihor.