Green’s Dictionary of Slang

(US) a cigarette, a cigar, a cigarette butt.

1905		‘West Point Sl.’ in Howitzer (US Milit. Academy) 292–5: Skag — Cigarette.
1912		Wash. Post 5 May 5/2–3: ‘Gimme a skag. Step out.’ [...] ‘Gimme a cigarette. Hurry up.’.
1918		Journal (Adelaide) 16 Jan. 1/6: ‘Kidney pudden, plum pudden, tin of custard, scags, pair of bonzo socks’.
1915		R. Bolwell ‘College Sl. Words And Phrases’ in DN IV:iii 235: scag, n. Cigarette stub.
1920		in A. Cornebise Amaroc News (1981) 29 Dec. 140: Buddie, lay off so many skags for a while.
1930		J.A. Russell ‘Colgate University Sl.’ in AS V:3 239: Scag: cigarette: ‘Pass me a scag.’.
1932		‘Patois of Annapolis’ in Sheboygan (WI) Press 17 Sept. 8/3: A cigarette is a ‘skag.’.
1957		(con. 1943) A. Myrer Big War 167: I’d give fifty bucks for a skag right now [...] Jesus, I could use a drag.