It's not a technical term, and the exact meaning can vary depending on context. Here's what it could mean:
* A gin and tonic: This is the most common usage, as a G&T typically has a significant amount of tonic water, which dilutes the gin.
* A weak gin cocktail: Any gin cocktail with a high proportion of non-alcoholic ingredients could be considered "thin."
* Gin that has been watered down intentionally: Some people prefer their gin diluted, and may intentionally add water to their drinks.
Essentially, "thin gin" is a term used to describe gin that is less concentrated and more watered down than it could be.