Derived from a small town in the US, silly is most often used as a noun to describe or act as a substitute for anything the speaker desires, but usually refers to somethingsexual in nature. Usually preceded by an article. (the silly)
"I can't find anything to write with. Go get me a silly."
"Put your clothes in the silly."
"They were doin' the silly last night!"
"Hey baby, wanna go do the silly? Eh eh?"
"What the silly??"
"What in the silly were you thinking?!"