There are plenty of synonyms for the phrase "not know personally." One could use the word "unacquainted," indicating a lack of familiarity with a certain individual. Another option could be to say "unsure of," which implies uncertainty or doubt. "Stranger" is another option, indicating that one has not met or interacted with a person before. Other words that could be used to convey a similar sentiment include "unknown," "foreign," and "mysterious." Ultimately, the choice of synonym will depend on the context in which the phrase is being used and the desired level of formality.