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Definitions of sure:

adjective:   physically secure or dependable
Example: "A sure footing"

adjective:   certain not to fail
Example: "A sure hand on the throttle"

adjective:   infallible or unfailing
Example: "A sure (or true) sign of one's commitment"

adjective:   (of persons) worthy of trust or confidence
Example: "A sure (or trusted) friend"

adjective:   exercising or taking care great enough to bring assurance
Example: "Be sure to lock the doors"

adjective:   impossible to doubt or dispute
Example: "Indisputable (or sure) proof"

adjective:   having or feeling no doubt or uncertainty; confident and assured
Example: "Was sure (or certain) she had seen it"

adjective:   certain to occur; destined or inevitable
Example: "Sudden but sure regret"

adjective:   capable of being depended on
Example: "A sure way to distinguish the two"

adverb:   definitely or positively (`sure' is sometimes used informally for `surely')
Example: "The results are surely encouraging"

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Alternative Terms
secure certain hard inarguable incontestable incontrovertible indisputable indubitable irrefutable positive unassailable undeniable undisputable unquestionable true-false-similar-words definite inescapable inevitable unavoidable infallible unerring unfailing sure-fire firm solid sound stable strong sturdy substantial unshakable continue-stop-pause fast-8116 steady assured confident undoubting sure-as-shooting sure-cure sure-enough sure-of-oneself for-certain make-sure slow-but-sure to-be-sure sure-brand unilever sure-mobile sure-every-little-thing-single eternity-every-little-thing-album sure-take-that-song take-that nobody-else sure-e2c canon-sure-shot50u sure-fit-daybed-covers suresby dicey oversure root-of-orchis kingdon-frank tancred hepatitis-g-treatment bacillary-angiomatosis-prevention power-down undecided sure-footed borderline fluky leery