Colorless Green Ideas

Colorless green ideas sleep furiously is a sentence composed by Noam Chomsky in 1957 as an example of a sentence whose grammar is correct but whose meaning is nonsensical. The phrase suggests that grammar is not the only principle underlying language, as was thought at the time. It indicates that words are symbols with associated properties that will not function if they are not used in the proper semantic context. By demonstrating that "meaning" is not dependent on the grammar of each localized language or culture, Chomsky's work established the theoretical basis for machine translation and generative grammar (the study of a "species-innate" cognitive structure that underlies language development).

Source (yeah, I've regressed to beginner-level academese, so that wasn't me talking) and where to read more: http://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_furiously

As a registered Linguistics Nerd, I have always found this brilliant and exciting.
The year before I "joined" it, the Linguistics Club at W&M had made a t-shirt illustrating this idea. Oh, how I coveted those t-shirts.

Nerd.