Etymology: "main" meaning sea or ocean

In Kipling's "The Land" he writes:

Then did robbers enter Britain from across the Northern main

And our Lower River-field was won by Ogier the Dane.

Here "main" seems to mean sea, i.e. the North Sea.

This seems to me to allude to the Spanish Main, but from what I can find that phrase actually refers to the mainland, not the sea.

The OED (1st ed) has "4. Said of a considerable, uninterrupted strech of land or water" (link) but the examples given mostly seem to be land.

Webster's 1913 has "main, n. 3 ... the high sea; the ocean" (link), but again the examples seem mostly to be of land, including "the main of Spain" to mean mainland.

etymonline doesn't have this usage of "main"

What is the etymology of this usage? Is it a corruption from "Spanish Main" coming to mean sea instead of "mainland"?

Solution 1:

OED3 has sense 5:

5. a. Short for main sea n.; the open sea. Now chiefly poet.

1579 T. North tr. Plutarch Liues 472 The winde stoode full against them comming from the mayne [Fr. le uent se tourna du costé de la pleine mer].

The meaning of main and main sea is made clear in the French translation pleine mer, but it's not clear where that translation is actually from. Although main meaning "sea" is still in use, albeit poetical, main sea is marked as obsolete, with the last citation from 1876: