The word insalubrious came into English in the seventeenth century from the Latin word insalubris, a combination of in-, meaning "not," and salubris, meaning "healthful." That makes for a decidedly unhealthy combination, and the word even has a slightly disreputable sound to it, like you’ve been out doing something you shouldn’t be doing. But at the same time it’s slightly formal, in part because it’s somewhat unusual in modern usage. Use it as a fancy way to describe something unhealthy that’s also a little bit common — like Las Vegas or fried chicken.