= infallible, adj. 2a.

Of persons, their judgements, etc.: Not liable to be deceived or mistaken; incapable of erring.

Incapable of erring; not liable to err; exempt from the possibility of error; infallible, unerring.

= inerrable, adj.

That does not err; free from error; unerring.

Quite certain, sure, unerring. (Cf. dead certainty in sense A. 31a.) dead shot, one whose aim is certain death; so dead on the bird. dead-on…