Bar L, n. — Green’s Dictionary of Slang

Bar L n.

[abbr. of proper name, but note the poss. pun on use of Bar in many brands used by US ranchers to distinguish their cattle, e.g. Bar X, Bar Y (in which the bar is a horizontal line drawn beneath the letter); Bar L thus offers a suggestion of the Wild West, in this case of Scotland]

(Scot.) Barlinnie Prison, Glasgow.

1989		J. Morton Lowspeak.
1993		I. Welsh Trainspotting 20: He’s obviously been residing in one ay the Windsor group hotels; Saughton, Bar L, Perth, Peterhead, etc.
1999		I. Rankin Dead Souls 76: Hardmen from Glasgow’s Bar-L.
2009		L. McIlvanney All the Colours 320: Maitland stays alive and out of the Bar-L.