I'm speaking about for example:
To mean: He's very pro/skilled.
I've seen this a lot, but I don't quite see what the origin of this comes from.
It's homophonic of 溜, which means proficient, smooth (in doing something) in Nothern dialects. Note it doesn't imply a top degree of mastery. It's more like nice instead of pwned.
The use of number 6 first appeared in online games, then spread to internet contents.
Some examples for the original word:
他爬树很溜（儿）。He's good at climbing trees.
他说的很溜（儿）。He gave a fluent speech.
One reference I can find: http://hs.tgbus.com/wenda/201410/20141002121649.shtml
I guess it comes from the dialectal pronunciation of 牛 which has the slang meaning "awesome".
In Chinese, there are many number combinations used as an abbreviation such as 666, 88 and 2333. Today I will introduce you 666. 666, in web terms, means 牛牛牛 or 溜溜溜 in Chinese. It is usually used to describe someone or something excellent or amazing.
Actually, 666 has been generated for a long time (the specific time is not clear). And it became popular and was used widely in a game named LOL (League of Legends). For example, if someone plays quite good in a game, you can say 666 to him.
Except for 666, there are also some other abbreviations that we use often in daily life or games.
1). 88: it means bye bye.
2). 5555: as the pronunciation of 5 is similar to the sound of crying, we usually use 555, 5555, 55555 to describe our sadness.
3). 520: it means “I love you”.
4). 5201314: it means “I love you for a whole life”.
5). 1314: it means “one’s whole life”.
6). 888: it can be used as a blessing, which shows your good wish of wealth.
7). 2333: this number combination is originated from a forum (www.mop.com). The No. 233 emoji from the MP forum is a picture of laughing, so many users like to add a “2333” when they post in the forum. Now “2333” is a popular web term used to express laughter. Resource from Sino5000