Totally O-fucking-holy-goddamn-shit-K.
Basically "OK" with more trash talk stuffed in between, making it 100% more OK per OK.
A meme derived from memoticon "OJ8K", which is simply a guy (or more commonly, a panda with memetic face) with an OK hand sign saying "OJ8K". "J8" is a euphony for "penis" in Chinese, in this case, it is used as a strong emphasis, similar to "fucking" or "bloody" in English. Simply put, OJ8K = O-fucking-K.
When the meme got more popular, people started to switch "J8" to various things such as different profanity words, expressions only found in particular dialects, internet slang, or even completely nonsensical gibberish. It doesn't matter what you put in between O and K, as long as you put something in, it'll make it more OK. Create your own.
For those who are curious, Mokou's OK here contains 2 internet slang and 1 expression in Sichuan dialect.
你妹 (your sister) is a strong slang expressing negative attitude and denial. Kind of similar to "my ass" in English.
e.g.
A: KFC for lunch?
B: KFC your sister, Subway FTW.
蛇皮 (snake skin) is a slang allegedly created by former pro-LOL-player PDD, originally used to describe a champion's Z-shape moving pattern for skill shot dodging. Common use is similar to original meaning.
e.g. In PUBG
A: Beware, someone may try to ambush us on that bridge.
B: No problem, with my snake skin driving, they can't even graze us.
棒棒 is allegedly a misheard word, mostly likely to be the last part of the famous insult in Sichuan dialect "日你先人板板 (Fuck your ancestor's coffin)."
