Origin and history of mo

Old English mara "greater, relatively greater, more, stronger, mightier," used as a comparative of micel "great" (see mickle), from Proto-Germanic *maiz (source also of Old Saxon mera, Old Norse meiri, Old Frisian mara, Middle Dutch mere, Old High German meriro, German mehr, Gothic maiza), from PIE *meis- (source also of Avestan mazja "greater," Old Irish mor "great," Welsh mawr "great," Greek -moros "great," Oscan mais "more"), perhaps from a root *me- "big."

Sometimes used as an adverb in Old English ("in addition"), but Old English generally used related ma "more" as adverb and noun. This became Middle English mo, but more in this sense began to predominate in later Middle English.

"Take some more tea," the March Hare said to Alice, very earnestly.

"I've had nothing yet," Alice replied in an offended tone, "so I can't take more."

"You mean you can't take less," said the Hatter: "it's very easy to take more than nothing."

As a noun, "a greater quantity, amount, or number," in Old English. More and more "larger and larger amounts" is from 12c. More or less "in a greater or lesser degree" is from early 13c.; appended to a statement to indicate nearness but not precision, from 1580s. The more the merrier "the larger the company the greater the enjoyment" is from late 14c. (þe mo þe myryer).

mid-13c., seur, "safe against attack, secure, out of danger," later "reliable, fit or worthy to be depended upon" (c. 1300); "mentally certain, confident of one's positions" (mid-14c.); "firm, strong, resolute" (c. 1400). It is from Old French seur, sur "safe, secure; undoubted, dependable, trustworthy" (12c.), which is from Latin securus "free from care, untroubled, heedless, safe" (see secure (adj.)).

The pronunciation development is that of sugar (n.). The colloquial pronunciation "sho" is attested by 1871 in representations of U.S. Black speech (fo sho); compare mo.

As "certain to be or happen," 1560s. As an affirmative meaning "yes, certainly" it dates from 1803. This traces back to the Middle English meanings "firmly established; having no doubt," and to phrases such as to be sure (1650s), sure enough (1540s), and for sure (1580s).

To make sure "establish beyond doubt" is from 14c.; be sure "be certain, do not fail" (colloquial, be sure to click the links) is by 1590s. Figurative formulas of certainty beginning as sure as are attested from late 14c. (as fire is red); as sure as (one is) born is from 1640s.

The use as an adverb meaning "assuredly, certainly" is from early 14c. Sure thing "something beyond the possibility of failure, a certainty," is by 1836.