Green’s Dictionary of Slang

(drugs) enjoying the immediate effects of a narcotic injection by injecting heroin little by little.

1966		J. Mills Panic in Needle Park (1971) 101: Instead of shooting the fluid in immediately he squeezed in a few drops, let it back up into the eyedropper again, squeezed in a little more, let it back up, squeezed more, and continued the in-and-out process until the fluid in the dropper was dark red with blood. The technique, known as ‘booting,’ is believed to prolong the drug’s initial effect.
1969		R.R. Lingeman Drugs from A to Z (1970) 49: booting [...] The technique of injecting heroin intravenously a little at a time, letting it back up in the eye dropper, injecting a little more, letting the blood-heroin mixture back up, and so on. The addict believes that this technique prolongs the initial pleasurable sensation of the heroin as it first takes effect.
1981		D.E. Miller Bk of Jargon 340: booting: See back-up.
1992		S.L. Hills Tragic Magic 8: There was a thing we called booting after you got a hit. A hit is when you get the needle in the vein and booting is allowing the blood to come up and mix with the dope in the eyedropper and pushing this mixture back in the vein several times.
1993		E. Richards Cocaine True 65: I’m being careful by putting it in little by little. It’s called booting.