Little Sister Syndrome 

In which a guy tells a girl who's obviously got the hots for him that he "thinks of her as a sister" or "she's a little sister to him," so as to blow her off lightly.
This is sometimes considered offensive and rarely considered an actual compliment, as the girl usually knows that this means that he doesn't see her as date-able.
Used frequently on friendly yet ugly girls.

Sam: Oh, Bertha, you're so cute!
Bertha: Really, you think so?
Sam: Oh yea; you're like a little sister to me!"
Bertha: Oh...

*later*

Sally: Sam complimented you?
Bertha: Yea, but then he ruined it via little sister syndrome.
Sally: Jerk.

Little Sister Syndrome by andi844 August 22, 2007

little sister syndrome 

When the younger sister of an older sister is more attractive than the latter

Laura is hot but her sister Rachael is infected with little sister syndrome.

little sister syndrome by 4pumpwonderchump July 15, 2009

little sister syndrome 

when a girl feels like she annoys everyone she talks too, the fear of being seen as an irritating burden

Sarah has an awful case of little sister syndrome, she wont tell me what’s going on

little sister syndrome by cemeterydoll November 30, 2022