Green’s Dictionary of Slang

1. in prizefighting, a blow (? a straight jab).

1857

Bell’s Life in Victoria (Melbourne) 5 Sept. 3/1: Tom caught him a severe straightener on the snuff-box, drawing lots of claret.

2. (Aus./Irish) a reviving drink; a drink that renders an alcoholic ‘normal’.

1902		Bulletin (Sydney) 16 Aug. 35/1: ‘Take a straightener, [...] and, for God’s sake, make haste.’ / ‘Is she very ill?’ inquired Clive, pouring brandy into a tumbler.
2021		P Howard Braywatch 6: I had to drive to the house at seven o’clock one morning so she could have her first straightener of the day.

3. an argument that may escalate into an physical fight.

1974		G.F. Newman You Flash Bastard 21: In a straightener Jack Manso would have little chance against either Terry Sneed or Peter Rosi, as rationality could always defeat irrationality.
1995		J. Cameron Vinnie Got Blown Away 15: Never needed to go looking for a straightener. Stand still it’d find me. Stand still too long and get no feet to stand on.
2001		K. Sampson Outlaws (ms.) 70: I would’ve rather had a straightener with him than take any swag back.
2007		N. ‘Razor’ Smith Raiders 311: Lenny [...] would offer a straightener to anyone.

4. an act of punishment, retribution.

2003

N. Griffiths Stump 9: Yeh don’t send two of yer best boys off on a straightener in a fuckin shed like this, lar.

5. a bribe.

1955		B. Hill Boss of Britain’s Underworld 185: A handful here and there and the straighteners would be well home. You can buy a cozzer down there [i.e. Tangiers] for fivepence.
1959		F. Norman Fings II i: Ask ’im ’ow ’e wants it, tools or a straightener.
1977		D. Powis Signs of Crime 203: Straightener Money, blackmail payment or a bribe.