(Ish in this case is not used to mean anything in particular, except as an addition to help the sound of the phrase - when used as a adjective - to flow into the next word.)
It is usually used to describe objects or persons in a positive sense, but a further definition of the word is that it describes someone or something possessing certain characteristics found in video-game bosses.
The boss of a video game comes at the climax of a level and has the player hyped, since they have been fighting lesser grunts to get to this stage.
The boss is considered to be more intimidating than his minions and is often accompanied by heavy toned, evil, ominous, and oppressive music (boss tune)
The boss is usually a dark and evil character, but it is the dramatic, stand out, dominating nature of a boss that can be implied by the use of "boss" as an adjective:
A character that is considered "boss" or "bossish", does not always have to be evil; he/she or it, may simply be a dominating, "badass" or antihero
The criteria for this is varied, but it is not often that you will see a cute character, such as a Pokemon described as "boss" or "bossish", unless it is in a sarcastic or ironic sense.
Hero characters of films can be known as "bossish" if played seriously, a camp character, like Flash Gordon is unlikely to fit this term, though in an ironic/sarcastic way it is used frequently.
The term itself has camp, comedy value anyway...
"Shut up fool, he ain't as bossish as Wolverine!"
"Huh? Bossish Wolverine?"