two by four, n.² — Green’s Dictionary of Slang

two by four n.²

also five to four, six to four, sixty-four

[rhy. sl. = SE whore]

a prostitute.

1935		A.J. Pollock Und. Speaks.
1946		St. Vincent Troubridge ‘Some Notes on Rhyming Argot’ in AS XXI:1 Feb. 46: forty-four. A whore. (Origin uncertain, but probably American.) Almost certainly American. The English is five to four, or six to four, from the betting odds against horses.
1960, 1975		Wentworth & Flexner DAS 648/1–2: five to four – whore. [Ibid.] six to four, sixty-four – whore.
1978		Maledicta II:1+2 (Summer/Winter) 119: He gives both two by four and six to four for ‘whore.’.
2003		B. Dark Dirty Cockney Rhy. Sl.