Green’s Dictionary of Slang

1. (drugs) to achieve the somnolent, peaceful state that follows an injection of heroin.

1933		Ersine Und. and Prison Sl.
1949		N. Algren Man with the Golden Arm 59: That first fix had only cost him a dollar [...] and sent him coasting one whole week end.
1959		J.E. Schmidt Narcotics Lingo and Lore.
1969		‘Iceberg Slim’ Pimp 94: All I wanted to do was bang ‘H’ and coast.
1976		N. Thornburg Cutter and Bone (2001) 60: Mo was already coasting on her first downer of the day.
1999		E. Bunker Mr Blue 87: In the argot of the junkie, I was coasting on the nod.
2001		ONDCP Street Terms 6: Coasting — Under the influence of drugs.

2. (drugs) to feel exhilarated when using cocaine [Spears, Slang and Jargon of Drugs and Drink (1986), suggests ‘a slight overdose’].

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 120/1: coasting. The exhilarating sensation produced by cocaine.
1960, 1975		Wentworth & Flexner DAS.

3. to relax; to act in a relaxed manner.

1938		W. Winchell On Broadway 28 Dec. [synd. col.] ‘Why don’t you take it easier – you’re doing all right – try coasting.’ [...] The trouble with coasting is that you can’t always depend on the brakes.
1955		‘Hal Ellson’ Rock 29: The winos are coasting.
1969		Cressey & Ward Delinquency, Crime, and Social Process 807: Mellow people [...] exchange ‘joints’ with a close friend, ‘bum’ cigarettes for a while, then give joints away, all the time ‘maintaining one’s cool,’ and ‘coasting’ from party to party.
1971		B. Short Black and White Baby 302: I wasn’t pushing anymore, I was coasting. I had become impatient to put school behind me, to put aside all that kid stuff and move along.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 201: coasting, v. – doing easy time; no difficulties.
1999		R. Jacobs in San Jose Mercury News 11 May n.p.: Coasting (v) – Relaxing. We were coasting at Sara’s house until the party started.
2012		D. Spivey ‘If You Were Only White’ 100: It was difficult, however, to imagine athletes wanting to coast with so much hype injected into the contests.