List of Japanese Slang
B
BL - Boys love
Brothercon- Brother complex, to be in love with one's brother.
Byenara - Bye + sayonara.
D
Deka でか/刑事 - Slang for detective.
F
Fujoshi - Japanese nerd girls. A bit of a negative term since it literally means "rotten girl".
G
gkgb (gakugaku buruburu) - To shake with fear. ((((;゜Д゜)))ガクガクブルブル
Gunbatsu - Excellent, high-quality. Japanese pig latin for batsugun.
K
Kitaaa (キター) - Something shouted when something exciting happens. Literally "It has come." キタ━━━━(゜∀゜)━━━━ッ!!
ktkr (Kitakore 来たこれ) - Something said when one's hopes come true.
Kurisotsu クリソツ - Means "lookalike". It's basically Japanese pig latin for sokkuri which means the same thing. This term has fallen out of use.
L
Lolicon - Someone obsessed with little girls. Short for Lollita Complex.
M
Mad Movies - Basically a term for movies mashed up in amusing ways.
N
Neta ni maji res ネタニマジレス - Someone responding to a joke post without realizing that it was written as a lie or a joke. (Source)
O
Otaku - A term for Japanese nerds. Means "house".
Ossu 押っ忍/押忍 - A macho way of greeting someone. Often heard in dojo or other similar martial arts settings.
Otsu - Means "thanks". Short for "otsukaresama". It's typically represented by the kanji 乙 which means something different entirely, but has the same sound.
S
seme - In a yaoi relationship, this is the dominant partner. Short for semeru/attack. The reverse in this relation is an uke.
Shoboon ショボーン - Dispirited, dejected. ｼｮﾎﾞｰﾝ(´･ω･`) Source
Shotacon - Someone obsessed with little boys.
Siscon - Someone in love with their sister. Short for Sister Complex.
Sutema Otsu - Literally means "Thanks for the stealth marketing".
T
Tsundere - When a character acts standoffish tsuntsun before melting and being nice deredere.
U
uke - In a yaoi relationship, this is the submissive partner. Short for ukeru/receive. The reverse in this relation is a seme.
W
wktk (Wakuteka) - To anticipate something.
w or any number of wwwwwwww - Basically Japan's equivalent of LOL. Short for Warau 笑, the Japanese word for "laughter".
w - Short for the English word "Double" which is pronounced the same way in Japanese.