Green’s Dictionary of Slang

the crevice between the buttocks that is revealed when someone wearing low-cut trousers bends forward.

1994		J. O’Connor Secret World of the Irish Male (1995) 57: His monstrous pink buttocks were being forced upwards and were protruding above his waistline like tumescent pillows (‘The Dagenham smile’ this phenomenon is called on London building sites).
1996		A. Close Official and Doubtful 36: Hard hat and brickie’s bum?
1998–2009		L. Haines ‘Buttocks, thongs, and bullsh*t’ on The Register 🌐 The Brits among you will recognise this cultural phenomenon immediately as ‘Dagenham cleavage’ – a tantalising glimpse of butt crack as favoured by hairy-arsed builders.
1999		Guardian Friday Rev. 11 June 14: He had become huge – all belly and builder’s crack.
1999		Observer Rev. 12 Sept. 4: And it was embarrassing [...] those plumber’s cracks.
2005		K. Lette Stitch in Time 94: Next came a diamante cocktail dress, with a split from ankle to ass and a low dip in the back more commonly associated with ‘builder’s bum’.
2021		T. Pluck Boy from County Hell 304: [S]idearm tucked in his pants, next to a sunburned plumber’s crack.