In Thai “Jing jing” means “really true”, so for example “Aroy jing jing” means “that really is delicious!”.
The Tai Chi Newbie may jar a little when they are told that Gin is at the heart of Chinese internal martial arts. Others, typically with some background in Qigong, might say “oh yes, of course, Jing”. “Jin” is neither of those contexts, although the latter person may have been tripped up by the Wade-Giles Romanisation of Chinese, where Jin is actually spelt Jing, and Jing is spelt Ching. It all gets rather confusing. Better to stick with Pinyin!
Gin, of course, is an alcoholic beverage. Using Pinyin, “Jing” would translate as something akin to “essence”, including seminal essence. Jing here is one of the three treasures, which properly combined in Taoist alchemical process, enable us to become immortal and ascend to heaven (something of a contradiction in terms to most westerners – http://en.wikipedia.org/wiki/Taoism_and_death). The other two treasures are “Qi” (“Chi” in Wade-Giles) – “internal energy”, and “Shen” – “spirit/wisdom”.
“Jin” means something like “skill”, “competency”, or “force”, depending on the context. So we have “Ba Jin” – the “Eight Powers/Techniques”, well-known to all serious Tai Chi Chuan practitioners: Peng, Lu, Ji, An, Cai, Lie, Zhou, Kao. Jin is also referred to numerous times in the Tai Chi classics – for example, as the result of constant, repetitive, quality training: “a hundred times refined steel”. Funnily enough, I found another romanisation when googling this: “The mobilization of the chin is like refining steel a hundred times over.” (http://www.fourseasonstaichi.com/CLASSICS.html). Better to stick with Pinyin.
Another place we find Jin widely discussed is in the theory of pushing hands, where we (at least) find “Ting Jin” (“Listening skill”), “Hua Jin” (“Diverting skill”) and “Fa Jin” (“Emitting/exploding skill”). If we master those three, then we may say that we have attained some “Dong Jin” (“Understanding Jin”).
As Tai Chi Chuan practitioners, concerned with the development of martial competency, we are not much interested in Gin or Jing, although at least one of those is important in life. Jin is a key concept in all internal martial arts, and is not unique to Tai Chi Chuan. It represents the developing, deep-seated skill that is the result of continuous training across a set of relevant exercises. In that respect, it is closely associated with “Gong” (“Kung” in Wade-Giles) which is the work by which we get it. So – go work on your Gong, and let your Jin develop – Jing Jing!!