A janitor is someone who is responsible for maintaining a building, primarily through cleaning it. This was often regarded as a menial task.
In 1953, when Benny Russell told Burt Ryan that he worked at the Arthur Trill Building, the police officer asked him if he was the janitor. His partner Kevin Mulkahey added that Russell was "awfully well dressed for a janitor". (DS9: "Far Beyond the Stars")
When asked why he was uninterested in seeing Vic Fontaine and his holoprogram, Benjamin Sisko stated that he felt it was an inaccurate representation of historical events, because the program was colorblind. He said that in reality, his people could have been "entertainers, or janitors, but customers? Never."
Later, during an attempt to rob the casino and thereby eliminate a Jack-in-the-box within the program, Nog impersonated a janitor as a way to gain entry to the room containing the safe. (DS9: "Badda-Bing, Badda-Bang")