Green’s Dictionary of Slang

1. to vaccinate.

1899

J. Flynt Tramping with Tramps 251: Some blokes’ve been jagged [vaccinated] two or three times.

2. to assault with a knife.

1909

J. Ware Passing Eng. of the Victorian Era.

3. (US) to copulate with a woman.

1960

G. Bluestone Private World of Cully Powers 159: You could [...] jag a million Loreleis.

4. (drugs) to inject oneself or someone else [note jack (up) v. (1)].

1999		Observer 26 Sept. 11: The classic implements for cooking heroin prior to smoking or ‘jagging’, or injecting, it.
2000		N. Griffiths Grits 74: An ar yer gunner tell us that she never fuckin uses em [i.e. needles] ter jag smack erself.
2003		N. Griffiths Stump 130: Becca. You’ve gorrer do it for me. You’ve gorrer jag it.

5. as sense 4 but in non-reacreational medical use.

2012

D. O’Donnell Locked Ward (2013) 153: Draw one [i.e an injection of tranquiliser] up for him and jag him in the jacksie.