The word is composed of "Not", which means a need of something vital, e.g. food, shelter, or oxygen (it's in this sense narrower than its English cognate) or "emergency". For example, you'd say people after a flooding are "in Not". "geil" itself can only be poorly translated, it means anything from "horny" to someone being "hot/attractive" in itself (e.g. "Sie ist so geil sie macht mich geil." -- "She is so hot she makes me horny."), to "great/awesome", to an exclamation for such great events, and is very slangly. Unlike "horny" or "great" however it's regarded not exactly as insulting, but simply as unfitting for "proper adult" speech. If you used it with your boss, for example, he'd be taken aback. "Notgeil" essentially evokes the image of something that gives you extreme blue balls, that makes you salivate like a dog does over a steak, that you want to bang *right now* or you'll go insane. It's slangy^2 and usually more humorous than a naked "geil", but can also be used in a darker context if applied to a serious situation.