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1.
A man who either consciously or unconsciously is attracted to and/or wants to have a sexual relationship with his sister.
"Man that girl is hot. She looks just like my sister."

"Dude, why do you bring up your sister in every conversation? ESPECIALLY when it's involving a hot chick? You're a Siul. That's so gross."
by __zsasz__ March 27, 2009
 
2.
A person who worships and adores the musical artist Prince. They strive to be like him in every way, shape and form.
Did you see that guy in the ruffled white top with the purple crushed velvet blazer? He's such a siul! I think he's also wearing eyeliner??
by BabyFoodBobert March 20, 2009