[saalweachter]

I believe "Caesar" derives from the word "caesaries", which means "hair/curls/beard-hairs".

Romans at the time were using three names, the given name (Gaius), the family/clan name (Julius) and the cognomen (Caesar), which was originally a nickname that became hereditary to identify a particular branch of a family.

So, the emperor of Russia was called the tsar because Gaius Julius or one of his ancestors was nicknamed "Curly" or maybe "Beardy".

[bee_rider]

All the statues I’ve seen of him make his hair look not so curly, and they don’t show him having a beard. This open up the possibility that many royal titles are ultimately named after some Italian guy’s magnificent chest hair (which would be hard to capture in a statue).

[saalweachter]

I'm with you all the way, but I'm pretty sure cognomen had transitioned from nicknames to hereditary by Gaius Julius Caesar's time. Also, clean-shaven was a relatively new fashion in Rome -- Cicero, one of Caesar's political enemies and of the previous generation, had a speech complaining about how women these days liked pretty clean-shaven younger men, and not the robust full-bearded old patricians, like they should.

[bee_rider]

Hopefully nobody wrote down the true source of his nickname so I can plausibly continue believing…

[NeoTar]

Another fun idea which has been proposed is that it’s an ironic nickname - so rather than ‘Baldy’ (which is you see busts of Caesar is certainly plausible) people called him or his ancestors ‘hairy’.

[saalweachter]

Are you familiar with the Three Stooges?

[Suppafly]

Another less popular theory is that it's from caesus which means to cut, which would be interesting because the cesarean section, also known as a C-section, was named after Caesar so it'd be a bit of circular definition.

>because Gaius Julius or one of his ancestors

Sextus Julius Caesar is the first Julii Caesares according to wikipedia. I just love that the term for all of them is Julii Caesares.