Is that Latin or Greek? Never did know what it meant?
Latin adjectives can have a masculine, feminine or neuter form, to suit the noun they're paired with. Thus, bonus the adjective meaning "good" could be bonus, bona or bonum, which can be abbreviated to bonus-a-um.
By the way, there is no truth in the story that Quintus Groovius Saxo was the world's first hard bopper.