Qijing Shisanpian (棋经十三篇, The Classic of Weiqi in Thirteen Chapters) is an ancient Chinese work of the 10th or 11th century.
Its Chapter 12: Class (品格) describes nine grades of improvement in a very concise manner. Note that "Class" here includes his manners, behavior and personality as well as his playing skills.
" There are nine grades of Class in Go.
- 入神 Near superhuman
- 坐照 Spiritual integration, or Instant comprehension 
- 具體 Integrated knowledge
- 通幽 Deep understanding
- 用智 Uses wisdom
- 小巧 Some cleverness
- 鬪力 Fighting strength
- 若愚 Looks stupid 
- 守拙 Knows himself 
And there are countless other grades not worth mentioning here. ...."
 This is the most difficult to translate. In zen, 坐照 means self-reflection by sitting still and meditating. Another interpretation is "At the instant of sitting (坐) grasps the whole situation with illuminating (照) insight". Segoe Kensaku supports the latter in his books.
 There is a Chinese idiom 大智若愚, which means "Who knows well may look to be stupid, because he also knows how little he knows and doesn't speak much".
 守拙 as an idiom means "Easy life without seeking high offices, knowing his own limitations".
In the 20th century, these phrases were used by Asian professional organizations as a convenient way to show their respect to the long tradition of Go. For example, the Zhongguo Weiqi Hui in Taiwan still uses 1 'pin' (品) to 9 'pin' as their professional ranks, instead of 9 dan to 1 dan.
Of course, it is pointless to compare the meanings of these ancient expressions directly with playing skills of professionals of the 20th century, let alone those of this AI and internet age.
Wikisource has the original Chinese text.
The website of the Italian Go Federation (FIGG) hosts a different translation of all chapters in English (PDF).
Anonymous: This is really informative and elegant translation, though with few exception that I can't totally agree. So let me express some remark here in my broken English, and please correct me if I am wrong.
Chapter 12 was translated as
"There are nine mental levels into which players are distinguished. The first is called 'being in the spirit', the second 'seated in enlightenment', the third 'concreteness', the fourth 'understanding changes', the fifth 'applying wisdom', the sixth 'ability', the seventh 'strength', the eighth 'being quite inept', and the ninth and last 'being truly stupid'."
I don't think the eighth (ruo4 yu2 in Mandarin Chinese) and the ninth (shou3 zhuo1) should be translated into negative terms. These nine levels are for good players, so it can't be inept nor stupid.
ruo4 yu2 means 'seemingly stupid' literally. shou3 zhuo1 means 'protect one's weakness' literally. These are not bad terms. I guess these nine terms are all from some Taoist classics or other classics, but I am not sure their origin and implied meanings. Today ru4 shen2 zuo4 zhao4 (the first and second level) are still used to describe a person with great skill (not necessary skill of Go).
As far as I know originally the rank of Taiwanese Go professionals were actually these nine levels instead of dan. And the Hankuk Kiwon uses these nine levels to describe dan players http://www.baduk.or.kr/information/info_player_ko.asp (dead link, see archive -- note that the page seems to use a non-Unicode character encoding). See the hanja in the parentheses (two characters for each level). I think the eighth level and ninth level which are used to describe Korean 2 dan and shodan respectively should not be translated as inept/stupid.