adjective
1.
firmly settled or established
trying to find a sure footing on the rugged slope
Synonyms: fast, firm, secure, stable, staunch, strong; compare fast 4, stable 4
2.
free from doubt, hesitation, or fear
upheld a sure faith
Synonyms: abiding, enduring, firm, fixed, never-failing, steadfast, steady, unfaltering, unqualified, unquestioning, unshakable, unshaken, unwavering, wholehearted
Related Word: assured, changeless, constant, unchangeable, unchanging, uncompromising, unfailing, unvarying; certain, fixed, set
Contrasted words: insecure, uncertain, unreliable; feeble, infirm, shaky, unsound; doubtful, dubious, hesitant
Antonyms: unsure
3.
Synonyms: infallible 1, inerrable, inerrant, unerring
4.
Synonyms: infallible 2, certain, surefire, unfailing
5.
marked by unwavering assurance especially as to the rightness of one's views or actions
was sure he knew the answer
Synonyms: certain, cocksure, confident, positive
Related Word: assured, self-assured, self-possessed, self-satisfied; arrogant, cocky, pert; decided, decisive
Contrasted words: doubtful, hesitant, uncertain
Antonyms: unsure
6.
Synonyms: positive 3, certain, incontestable, incontrovertible, indisputable, indubitable, uncontestable, undeniable, unequivocal, unquestionable
Related Word: convincing, telling; absolute, definite; genuine, real, valid