There are several synonyms for the phrase "dead sure" which can be used in different situations. One such synonym is "absolutely certain." This phrase means that there is no doubt whatsoever about something. Another synonym is "guaranteed," which implies that there is a guarantee or assurance of something happening. The phrase "beyond any shadow of a doubt" can also be used as a synonym for "dead sure," signifying an unshakeable conviction or belief. Similarly, "convinced beyond a reasonable doubt" can also be used to indicate that one is completely sure of something. "Positive" and "confident" are other synonyms that can be used interchangeably with "dead sure" to convey a strong sense of conviction or certainty.