Explanation: After Julius Caesar became dictator of the Late Roman Republic after his civil war, his popularity and suspicions over his intentions clashed. On one hand, he was genuinely and wildly popular amongst the Roman people - and some of the aristocracy. On the other hand, the Roman people largely did not want a king to rule over them - they wanted someone who would ‘set the Republic right’, however long that vague task took, and then return to normal cutthroat Roman politiking.

So some sycophants of Caesar, many enemies of Caesar, and some who simply were tired of the Late Republic’s dysfunctionality, would all occasionally raise the issue of Caesar becoming king, each group with their own motivations for making Caesar seem kinglike.

In one notable incident, Caesar was hailed, as he was going through the street, as ‘Rex’ - ‘King’. Very dangerous - to rebuke one’s own supporters damages one’s popularity, but to allow oneself to be proclaimed a king, even informally, would be to prove all his enemies correct.

So Caesar made a joke of it. At that late point, the monarchy was so ancient that ‘Rex’ was also used as a name (in the same way we might know a ‘John King’ in the modern day) - so he light-heartedly responded, “Non rex, sed Caesar!” - “Not King, but Caesar!”, acting as though he had been mistaken for someone else instead of proclaimed a monarch.

There were a number of incidents like that during Caesar’s dictatorship, where he carefully maneuvered around the ambitions of his supporters and the fears of his enemies (and some neutral parties).