Etymology
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rad (n.)

1918, "x-ray dose unit," a shortened form of radiation (q.v.). The meaning "unit of absorbed dose of ionizing radiation" is by 1954, an acronym from radiation absorbed dose. As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang adjectival sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).

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Definitions of rad

rad (n.)
a unit of absorbed ionizing radiation equal to 100 ergs per gram of irradiated material;
rad (n.)
the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees);
Synonyms: radian
From wordnet.princeton.edu