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.

lil mad is like big mad but the opposite, you mad but not mad to the extent of pullin somthin

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.

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.

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

the Japanese word for "older brother" (younger brother is itouto).

Onee-chan is Japanese for older brother.