Green’s Dictionary of Slang

1. to embrace sexually.

[1728		J. Dalton Narrative of Street-Robberies 51: Falling in Company with one Moll Clincher, at a Brandy-Shop, she persuaded him to go with her to Chelsea, where she would entertain him].
1899		Ade Fables in Sl. 26: George was sitting on the Front Porch with a good Side Hold on Mabel [...] While they were Clinched, Mabel’s Father [...] came down the Stairway and out to the Veranda.
1902		Ade Forty Modern Fables 96: All the Married Couples would Clinch at the Hotel Steps.
1929		H.C. Witwer Yes Man’s Land 100: A lot of close-ups was took of Calhoun in a football outfit clinching with Fay.
1938		W. Smitter F.O.B. Detroit 37: ‘My snookie-um-u-m-m-ums.’ They clinched.
1960		G. Swarthout Where the Boys Are 178: Tuggle and Quentin clinching.
1965		J.P. Carstairs Concrete Kimono 36: We clinched.

2. to catch, to arrest.

1852		Kendal Mercury 17 Apr. 6/1: But at last he vas clinched, and bound fast in the start.
1895		Crowe Aus. Sl. Dict. 18: Clinch, to be imprisoned.

3. to shake hands.

1896

F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 81: An’ with that they clinched.