Derogatory phrase referring to a woman you would be embarrassed to be seen with. As in, you want to ride it but you don't want your friends to see you on it.
Guy:I'd like to have sex with that voluptuous lady over there but my friends only think skinny chicks are hot.
Friend: Yah, she's your sister's bicycle.
Friend: Yah, she's your sister's bicycle.
by GaeaKali November 30, 2014