Jack (n.): plainclothes policeman, detective. ‘“Jack”, plain clothes policeman’ (Jones 1935: 5). ‘You just call in that jack that's lookin's after you and see what he's got to say’ (Jerome 1948: 25). ‘Dodging the Jacks (detectives), the Scuffers (Police, Slops, also being used) or their friends’ (Shaw 1958d: 16). ‘Detectives. Jacks, Old Bill, Plainees’ (Minard 1972: 89). ‘Black and white, a lot of Uniform work is, not bloody grey like the jacks have to deal with’ (McClure 1980: 83). ‘Jack C.I.D. Officer’ (Spiegl 1989: 48). ‘Wrecked the few brain cells the fuckin jacks left intact’ (Griffiths 2003: 31). ‘Jack – policeman, man, fellow’ (Callaghan 2011 [1920s–30s]: vii). Recorded from m.19c.; derivation unclear, though there is a long history of ‘jack’ being used in a derogatory or contemptuous sense.

Jack (something) in (v.): to give up, renounce, finish. ‘Beer mannan Jack! Stick to your principles! Damn the job!’ (Minard 1972: 36). ‘The only real escape is to jack in yer job, walk out’ (Griffiths 2003: 39). Recorded from m.20c.; an extension of m.19c. ‘to jack (something) up’, ‘to give up suddenly’.

Jackie/jacksharp (n.): stickleback. ‘Chasing poor little fellows, because they were watching the jack-sharps play’ (Shimmin 1863: 118). ‘They were going to fish for Jack Sharps’ (Hanley 1932: 123). ‘They are called jacksharps … Of course the word is cut down to “Jackie”’ (Shaw 1960a: 41). ‘We wen ter Sevvie ter ketch jackies. We went to Sefton Park to catch small fish (Jack Sharps)’ (Shaw et al. 1966: 46). ‘Jackies. Tiddlers. Cf Jacksharps. Liverpool’ (Howarth 1985: n.p.). Recorded from e.19c.; ‘still current in Liverpool, mid c.20’ (Beale (1984) s.v. jack sharp); a northern dialect name for the stickleback.

Jam butty car (n.): police car. ‘The department's “jam butty” cars are distinguished by the red stripe across their sides’ (McClure 1980: 19). ‘JAM-BUTTY CAR Old type of police car’ (Spiegl 1989: 48). ‘The “jam butty” patrol cars’ (Lees 2013: 166). *NR; from the red horizontal stripe that ran along the side of the white car.