Etimology
From Middle English sure, seur, sur, from Middle French sur or Old French seür, from Latin sēcūrus (“secure”, literally “carefree”), from sē- (“apart”) + cūra (“care”) (compare Old English orsorg (“carefree”), from or- (“without”) + sorg (“care”)). See cure. Doublet of secure and the now obsolete or dialectal sicker (“certain, safe”). Displaced native Middle English wis, iwis (“certain, sure”) (from Old English ġewis, ġewiss (“certain, sure”)), Middle English siker (“sure, secure”) (from Old English sicor (“secure, sure”)) with which was cognate. This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term. When and how did the letter"s" come to be pronounced as "sh"? The OED entry points to a source that may have information on this: E. J. Dobson Eng. Pronunc. 1500–1700 (ed. 2, 1968) II. §185 (iii.) and $388. Perhaps early irregular yod-coalescence?
adjective
sure (comparative surer, superlative surest)
Physically secure and certain, non-failing, reliable.
Certain in one's knowledge or belief.
(followed by a to infinitive) Certain to act or be a specified way.
(obsolete) Free from danger; safe; secure.
(obsolete) Betrothed; engaged to marry.
Examples
This investment is a sure thing. The bailiff had a sure grip on the prisoner's arm.
The Celebrity, by arts unknown, induced Mrs. Judge Short and two other ladies to call at Mohair on a certain afternoon when Mr. Cooke was trying a trotter on the track. The three returned wondering and charmed with Mrs. Cooke; they were sure she had had no hand in the furnishing of that atrocious house.
Roy: I'm 95% sure it was him.Jen: You... you said you were 99% sure.Roy: I'm 97% sure it was him.
He is sure she was lying.
He was sure of being a finalist.
They aren't completely sure who will attend.
You seemed sure that the car was his.
Be sure to lock the door when you leave.
Feare not: the Forreſt is not three leagues off, / If we recouer that, we are ſure enough.
The king was sure to Dame Elizabeth Lucy, and her husband before God.
I presum'd […] [that] you had been sure, as fast as faith could bind you, man and wife.
Related words
synonyms
(secure and steadfast): certain, failsafe, reliable, sicker
(steadfast in one's knowledge or belief): certain, positive, wis
adverb
sure (comparative more sure, superlative most sure)
(modal adverb) Without doubt, certainly.
(archaic) Without fail, surely.
Examples
Sure he's coming! Why wouldn't he?
"Did you kill that bear yourself?" ―"I sure did!"
These high and gusty relishes of life, sure,Have no allayings of mortality in them.
Theſe are the wings ſhall make it flie as ſwift,As dooth the lightening: or the breath of heauen,And kill as ſure as it ſwiftly flies.
interjection
sure
Yes, expressing noncommittal agreement or consent.
Yes; of course.
You're welcome; polite response to being thanked.
Examples
"Do you want me to put this in the garage?" "Sure, go ahead."
"Could you tell me where the washrooms are?" "Sure, they're in the corner over there."
"Thanks for helping me with that electrical fault." "Sure. Any time."
Related words
synonyms
(noncommittal yes): OK, yes
(response to being thanked): of course