"x-ray dose unit," 1918, shortened form of radiation (q.v.). As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).
rad 1 (rād)n. A unit of energy absorbed from ionizing radiation, equal to 100 ergs per gram or 0.01 joule per kilogram of irradiated material.
rad 2abbr. radian
An acronym for radiation absorbed dose. A unit of energy absorbed from ionizing radiation.
Extraordinary; wonderful; awesome, chill, gnarly: Want to go to this way rad party? (late 1970s+ Teenagers)
A radical (1820+)