A Japanese term made of these parts:
1. The 'o-' prefix, indicating a respectful tone.
2. 'nee', meaning 'sister'.
3. The '-sama' suffix, an honorific denoting extreme
respect as might be used when addressing noble figures or gods.
Onee-sama is an unusual, overly formal way of referring to one's sister. For most everyday purposes, the neutrally formal 'onee-san' or familiar 'onee-chan' would suffice: only those raised in highly formal settings would even contemplate using 'onee-sama' in a non-sarcastic way (as one might say in English '
oh of course your highness').
'Onee-sama' is a term mostly popularised by manga and anime, specifically shoujo-ai (love between young
girls) series set in
high-class boarding schools (see Strawberry Panic for a textbook example). Because of this association, 'onee-sama' is inextricably tied to the more elegant side of the shoujo-ai genre: a young
girl using the term to address an older, unrelated
girl could be hinting at a deeper
relationship between the two of them. Or they could just be messing with you.
IRL (sort of): "Onee-
sama, I hope you aren't labouring under the misapprehension that your seniority over me justifies you putting on airs."
Shoujo-ai fantasy: "Onee-
sama, would you
please become my soeur?"
