The cmavo {zo'o} literally means "humourously", and like all cmavo of it's class ("UI"), it is scalable- depending on the suffix-like cmavo attached to it can mean anything in the range between "very humourously" and "very seriously", with the midpoint of the scale being "dully", as such:
zo'ocai - very humourously
zo'o - humourously
zo'ocu'i - dully
zo'onai - seriously
zo'onaicai - very seriously
For more info, you can go to the Vocab section of the Lojban site- there are other resources as well.