What does the letter 'u' mean in /dev/urandom?

Unlimited.

In Linux, comparing the kernel functions named random_read and random_read_unlimited indicates that the etymology of the letter u in urandom isunlimited.

This is confirmed by line 114:

The /dev/urandom device does not have this limit [...]

Update:

Regarding which came first for Linux, /dev/random or /dev/urandom, @Stéphane Chazelas gave the post with the original patch and @StephenKitt showed they were both introduced simultaneously.

It depends on which "Unix" system you are talking about.

On FreeBSD, /dev/urandom and /dev/random are the same device. The letter u is now a historical legacy that exists for backward compatibility. At startup, they block until enough entropy has been gather and then never block again. See Myths about urandom for details.

In the modern Linux world (starting with kernel 4.8), both devices pull from the same CSPRNG, so the only difference is that some people speculate at the existence of an attack. This attack is like FTL [Faster than Light] travel. Easy to speculate about, rather hard to actually design.

TLDR is just use /dev/urandom.