Green’s Dictionary of Slang

to tell a substantial lie.

c.1698

B.E. Dict. Canting Crew n.p.: Strain hard to ly heartily.

to urinate.

1984

Partridge DSUE (8th edn) 1164/1: [...] C.20.

1. of a man, to have sexual intercourse.

1984

Partridge DSUE (8th edn) 1164/1: earlier C.20.

2. to urinate.

1997

Roger’s Profanisaurus in Viz 87 Dec. n.p.: strain your greens euph. To have a piss.

(US) to strain oneself.

1909		in Journal of Amer. Folklore XXVI (1913) 128: Bought a cow of Farmer Jones, / She wasn’t nothing but skin and bones; / Kept her till she was fine as silk, / Jumped the fence and strained her milk [HDAS].
1935		J. Conroy World to Win 71: Take it slow and easy [...] Don’t strain your milk.
1938		J. Weidman I Can Get It For You Wholesale 291: ‘Tell Mr. Babushkin not to strain his milk.’ ‘What?’.

to urinate.

c.1864		‘So, I Said to Myself’ in Rakish Rhymer (1917) 48: Some time after that I my taters was straining, / When all of a sudden I felt a great pain in / My pri–tes.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 17: I’m bustin’ to strain the potatoes. [Ibid.] 85: Just got to strain the spuds.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xl 4/5: strain the spuds: Use the toilet.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 43: Strain the Potatoes Urinate.
1992		Sydney Morn. Herald 26 Sept. 162/2: One day, at smoko [...] Alex stood up, ‘Se you in a jiff, Bun, just going to strain the potatoes.’ And he ambled off in the direction of the [...] dunny.
2003		McGill Reed Dict. of N.Z. Sl.