onii an unknown Asian name that is some how in a few baby name books

TLDR: Older Brother

A little more detail if you like-
Onii can end with 3 different Japanese honourifics, -San, -Chan, or -Sama.
-San is the basic ending for the basic big bro.
-Chan is a more compassionate ending for Onii if you really love your big brother and feel emotionally attached to him greatly
-Sama is a bit of a formal ending for big brother, nothing too special.

Onii-Chan means "Big brother" in Japanese. Most people assume it's a mating call of some sort because of hentai but no that's just hentai. Lots of hentai has incest.

Onii-san is Japanese honorific, meaning 'Older Brother'. It is used within both families or close friends. The 'San' is used for a sign of respect by others within the family or outside of the family and the 'Onii' is to signify someone as a brother.

1. Used to refer to your older brother in a cute way (in japanese).
2. In hentai, mostly lolicon, the lolis use it to refer in a cute way to their lover.
3. Cringey term that lots of weeaboos use to refer to their crushes.

a respectful japanese term for 'older brother', 'onii' being brother, and 'sama' being a reverence of an ending.

To most normal people it means "brother" but...
if you're a nasty, it means "fuck buddy"