The sissy bar is a mild slang term for the user of the bar. Originally known as the sister bar as is you have your sister riding with you on your motorcycle because your mom is making you watch over her or something like that anyway... Interestingly, in the early 1960s, some states made a law mandating that bars be used for the passenger of the motorcycle for their safety. But old school bikers being old school bikers didn't like the idea and generated a nickname making fun of them (mostly because they didn't like having to obey the new laws)
Since then a sissy bar is used for a lot of practical things. Strapping sissy bars bags to it, you passenger can lean back on it, or even strap your leather jacket to it.