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1.
The suffix used in Japanese names when referring to "Big Brother." This is usually used in the family by the younger siblings but, can also be used by younger friends outside the family.
Katsuro-niichan

Domo Niichan (Thank you Big Brother)
by Cole Daniel November 14, 2007
 
2.
Japanese for younger brother.

- Chan for cute, a term of endearment.
"Nii-chan, where did you put the potatoes?"
by Alexandra M.G. March 07, 2008