"Keep on keeping on": We can do a bit better than
Cassell's Dictionary of Slang, which traces that phrase back to the 1910s. Consider Charles Haddon Spurgeon back in 1888: "I spoke about the difficulty of keeping on. 'Yes,' answered my friend, 'and it is harder still to keep on keeping on'" (
Spurgeon's Gold: New Selections from the Works of C. H. Spurgeon, p. 30).