This is used commonly to appear cute or to avoid questions that you dont want to answer
(can be annoying to some)

EXTENDED VERSION: Nico Nico-nii~ Anata no Heart ni Nico Nico-nii,~~
Egao todokeru Yazawa Nico Nico~ Nico-nii te oboeteru Love Nico~
Nico: Nico Nico nii~~
Kotori: Gosh, ur so cute, Nico!!
Nico: Teehee i sure am, Nico nii!!~~
Maki: Fuck off, Nico ur so annoying
Nico: Hmph

Nozomi: Nico, did you finish ur homework? You also have to study for the exam next week
Nico: ....
Nico: Nico~Nico~Nii~~~!!!!
Nozomi: ....
µ's: Awww Nico is so cute!!
Nozomi: Aww yeah she is
Nico: Nico!~~
Nozomi: What was i saying again?
Rin: Probably something about astrology nya~
Nozomi: Lmao ya probably tbh

In these examples, you can see that by using the Nico Nico nii you can get different results
by Stolen dance November 22, 2016
Serious definition: Nico Yazawa's catchphrase from the anime show Love Live! School Idol Project. Though this character seems mean and annoying at first glance, she uses this phrase often to cheer people up and show them that they don't need to feel sad, as they should always smile no matter the situation.
Nico nico nii~~ I'll nico nii your heart! I'm Yazawa Nico-Nico, and I'll put a smile in your heart! Remember, I'm Nico-Ni, and I love-Nico you!
by ZAPBoy October 21, 2019
The phrase mentioned by Nico Yazawa, a member of the idol group μ's (pronounced "muse") in the anime, "Love Live! School Idol Project". It can either sound really cute or really annoying to you. No in-betweens.
Nico: "Nico nico nii!!"

Umi: "stfu bruh"
by Syahrul Danial August 30, 2020
Every weeb phrase when meeting another weeb
Or

Cancer to everyone except weeb.
by Shajanrai March 31, 2017
nico nico nii
nico nico nii
by Mandy Mouse September 25, 2019
Its a meme and it’s a great meme but I don’t know it’s a meme and I also use it for something that is really cool
Person 1: did you see Jake help that falling kid

Person 2:yeah,he is such a nico Nico nii
by Di meme lord January 6, 2019
It is a Japanese term for "Say cheese"
It is a popular term amongst weebs as well
Person 1: "Hello can you take a picture of us"
Person 2: "Sure!"
Person 2: "Nico Nico Nii (Say cheese)"
by Sangwoo_w33b October 24, 2020