Green’s Dictionary of Slang

(orig. US teen) a general intensifier; extreme, excessive, very much, excellent, the best.

1977		L.A. Times 21 Sept. IV16/1: Radical: Getting crazy on your [skate]board; attempting something that’s difficult, such as riding vertical. Rad: Synonym for ‘Radical’ -- more commonly used than Radical.
1978		S. King Stand (1990) 665: ‘Rad’ and ‘gnarly’ were ways of saying a thing was good.
1984		Eble Campus Sl. Sept. 6: radical – unexpectedly wild [...] shortened form rad.
1990		J. Wambaugh Golden Orange (1991) 33: He bitched about missing some rad tubes.
1997–2001		Online Sl. Dict. 🌐 rad-o adj 1. excellent. (‘Whoa, this new album is rad-o!’).
2004		T. Dorsey Cadillac Beach 198: One part Maui-Wowee rad resin weed that makes you see the Devil.
2010		IOL News (Western Cape) 22 Oct. 🌐 That makes him a bad-ass. And that rad tattoo.
2017		T. Spicer Good Girl Stripped Bare 13: In Year 4, we analyse a song by Skyhooks. The lead singer, a man called Shirley, performs topless. how rad.
2022		I. Fitzgerald Dirtbag, Massachusetts 216: [T]he dude is so rad and unimpeachable .