Etymology
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sure (adj.)

early 13c., "safe against attack, secure," later "firm, reliable" (c. 1300); "mentally certain, confident" (mid-14c.); "firm, strong, resolute" (c. 1400), from Old French seur, sur "safe, secure; undoubted, dependable, trustworthy" (12c.), from Latin securus "free from care, untroubled, heedless, safe" (see secure (adj.)). Pronunciation development is that of sugar (n.).

As an affirmative meaning "yes, certainly" it dates from 1803, from Middle English meanings "firmly established; having no doubt," and phrases like to be sure (1650s), sure enough (1540s), and for sure (1580s). The use as an adverb meaning "assuredly" goes back to early 14c. Sure-footed is from 1630s, literal and figurative; sure thing dates from 1836. In 16c.-17c., Suresby was an appellation for a person to be depended upon (see rudesby).

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Definitions of sure
1
sure (adj.)
having or feeling no doubt or uncertainty; confident and assured;
was very sure in his beliefs
was sure (or certain) she had seen it
sure of her friends
Synonyms: certain
sure (adj.)
exercising or taking care great enough to bring assurance;
be sure to lock the doors
Synonyms: certain
sure (adj.)
physically secure or dependable;
a sure footing
was on sure ground
sure (adj.)
reliable in operation or effect;
a sure way to distinguish the two
wood dust is a sure sign of termites
Synonyms: certain
sure (adj.)
(of persons) worthy of trust or confidence;
a sure (or trusted) friend
Synonyms: trusted
sure (adj.)
infallible or unfailing;
a sure (or true) sign of one's commitment
sure (adj.)
certain not to fail;
a sure hand on the throttle
sure (adj.)
impossible to doubt or dispute;
indisputable (or sure) proof
Synonyms: indisputable
sure (adj.)
certain to occur; destined or inevitable; "In this life nothing is certain but death and taxes"- Benjamin Franklin;
sudden but sure regret
he is sure to win
Synonyms: certain
2
sure (adv.)
definitely or positively (`sure' is sometimes used informally for `surely');
he'll win sure as shooting
they thought he had been killed sure enough
it's going to be a good day for sure
they sure smell good
sure he'll come
Synonyms: surely / certainly / for sure / for certain / sure enough / sure as shooting
From wordnet.princeton.edu