Green’s Dictionary of Slang

(drugs) intensifying the effect of cocaine by heating it in combination with ether or other chemicals before inhaling it.

1980		Courier-Mail (Brisbane) 18 June 1/1: A police spokesman said Pryor told doctors he was ‘free basing’, purifying cocaine by using ether and a flame when he was burned [OED].
1981		Sepia 30:5 63: Freebasing is smoking cocaine that has been reduced to its purest form by cooking it in a soda and water mixture.
1985		N.Y. Post 8 Aug. 21: A powerful new form of cocaine – which produces the effects of ‘freebasing’ without its fiery risks – was discovered last night during a police raid on a Bronx narcotics den.
1997		L. Davies Candy 39: Lex had his own little problems, sometimes with heroin, sometimes with freebasing.
2003		(ref. to 1981) J. Lethem Fortress of Solitude 251: Shit, man, I was there at the birth, me and them Philly cats might of practically invented freebasing.