Infallible, positive, certain. †Also as adv.

Incapable of failing; sure, reliable.

Not liable to failure, defect, or decay; unfailing; that cannot fall short, come to an end, or be done away.

Of things: Not liable to fail, unfailing. Not liable to fail in its action or operation.

Incapable of being defeated or undone; indefeasible; indefectible.

(a) designating a reliable or accurate firearm; spec. designating a gun which reliably discharges blanks, used as a stage prop (now rare); (b)…

(un-, prefix1 affix 1b.)

certain to succeed or attain the desired end, guaranteed; cf. surefire adj., shot, n.1 10c.

North American colloquial. Certain to succeed or achieve the desired outcome; guaranteed.