Green’s Dictionary of Slang

1. a bar (in a public house).

1960		J. Franklyn Dict. of Rhy. Sl.
1971		J. Jones Rhy. Cockney Sl.
1977		D. Powis Signs of Crime 189: Jack tar A bar, of licensed premises: ‘He’s in the Jack.’.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.

2. ten shillings [= half a bar under bar n.¹ ].

1969

S.T. Kendall Up the Frog 34: Jack Tar – Half a bar.

3. a bar (in a jail cell).

2002

D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 One thing I did know for sure and that was that Lingers was doing a long spell in one of Her Majesty’s flowery dells with a lot of jack tars in the window.