Top Definition
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...


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

by andi844 August 18, 2007
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.
by 4pumpwonderchump July 15, 2009
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