Plain and simple, 'Robana' is a combination of two words. 'Rotten' and 'banana.'
Like most colloquial terms and combinations, 'Robana' makes saying or describing something something different, more fun, and a little more easier.
It is commonly used as a descriptive word through metaphors and similes.
One could refer to someone or something as a 'robana' in a metaphorical sense, or, to make the simile, one could simply describe someone or something's stench--or whatever, really--to be like that of a 'robana.'
Tyler smelled like a robana in class after having come from the blistering Texas heat outdoors.