I. noun
: something that is or is held to be certain (as to succeed or bring success) : something reliable in behavior or development : something (as a bet) upon which one allegedly cannot lose
a positively sure thing in a race — Joel Sayre
a sure thing does not arouse us emotionally — John Dewey
II. adverb
: certainly — often used interjectionally
III. phrasal
: something safe to wager on : something certain to take place : a contestant certain to win