rad

The word 'rad' has a dual origin. In its informal, adjectival sense meaning "excellent," it emerged in American youth slang during the late 1970s as a shortened form of "radical," which itself had evolved from its political meaning to denote something extreme or cool. Its scientific use as a noun, a unit of absorbed radiation dose, originates from the mid-20th century as an acronym of "radiation absorbed dose." The two meanings are homographs, sharing spelling and pronunciation but arising from completely independent linguistic processes—one through colloquial abbreviation and semantic shift, the other through technical acronym formation.

This word is a simplex form with two distinct etymologies. As slang, it is likely a clipping of "radical." As a scientific unit, it is an acronym.