Green’s Dictionary of Slang

1. a partially smoked cigarette.

1984		Partridge DSUE (8th edn) 331/1: —1935.
1980		Barltrop & Wolveridge Muvver Tongue 12: ‘Doofer’ for a remnant of cigarette, pinched out and put aside for later: it will do for later on.
1989		R. Jolly Jackspeak [ebook] doofer [...] a cigarette that has been half-smoked, extinguished, and then carefully stowed inside Jack’s cap as something that will do fer later.
1996		R. Spears Sl. American Style 106: doofer AND dufer [ ' duf? ] n . a ( found or borrowed ) cigarette saved for smoking at another time . ( It will ‘do for’ later.) Sam always has a doofer stuck behind his ear . He takes two fags , one to smoke and a dufer.
2014		(con. WW1) Leighton Buzzard Writers Bridge Between Two Worlds 113: Ford took one last drag on his minute cigarette — what we called a ‘doofer’ — cigarettes that would ‘do for now’- one part baccy and nine parts trench, or at least that’s what they smelt like.
2023		(con. WW2) F. Farrell Deck [ebook] A cigarette, a doofer, half smoked, tucked behind one ear for later.

2. (Irish, also doofah) any otherwise unnamed object.

1979		J. Morrow Confessions of Proinsias O’Toole 75: He held it, cocked, level with his right little-red-eye ... ‘Mustn’t forget your wee doofer, Francie,’ he said, the bastard.
2000		Indep. Rev. 8 June 11: Brian could play all manner of sitars and ‘tinkly-bonk doofahs’.
2009		T. Black Gutted 41: Mac took the doofer [i.e. TV channel selector] off me.