ut stands for Ultra Terminal. That was a very early Engineering guess at a product name, back in the mists of time when the project needed to start reserving software package names (and, consequently, prefixes for executable names). Most of the Sun Ray packages have SUNWut* names. The ones that don't are following some other established convention: for instance, Sun Ray's message localisation packages have their own naming style which predates Sun Ray.Also from the same page:
I was told it was 'universal terminal'. Older versions used to have a newt display at certain points during Sun Ray boot up - newt was apparently a codename for a while too. After that it was called 'Corona', then they settled on Sun Ray.