What is "that thou fear'st"?

Editors generally assume that Olivia assumes that Cesario is denying that he is her betrothed husband because he is afraid of Orsino. If this is true, Olivia is telling Cesario that if he asserts his status as her betrothed husband, he is therefore a Count (as she is a Countess) and has nothing to fear from a Duke (Orsino).

However, "what thou fear'st" may simply be the status of being Olivia's husband. Olivia may be assuming that Cesario doesn't want to be a social climber. (One of the points of the practical joke played on Malvolio is that social climbing is odious.)