Green’s Dictionary of Slang

thirteen adj.

SE in slang uses

In compounds

In phrases

thirteen-button salute (n.) [the thirteen buttons on USN trousers, which are pulled undone to reveal the genitals]

(US navy) sexual intercourse.

Military Terms in http://www.combat.ws 🌐 thirteen-button salute: euphemism for when a sailor pulls down sharply on the upper corners of the fall front on his uniform breeches so as to quickly and fully expose his genitals, often to accommodate a powerful urge to urinate copiously, or to immediately satisfy a lustful desire.
[Aus]D. Whish-Wilson Shore Leave 24: [P]retending they were real sailors, lying through their teeth as they angled for a thirteen-button salute with a local girl.
thirteen clean shirts (n.) [prisoners were allotted one clean shirt every week]

(UK prison) a three-month sentence.

[UK]N. Devon Jrnl 8 Feb. 7/2: To get three months hard labour is more pleasantly described as getting thirteen clean shirts.
Ithaca Jrnl (NY) 14 Oct. 10/1: In the 19th century, slang expressions such as [...] ‘thirteen clean shirts’ (three months in jail at the rate of a clean shirt per week) reinforced the belief that the number lacks charm or luck.
[UK]Farmer & Henley Sl. and Its Analogues.
[Aus]Oakleigh Leader (Nth Brighton, Vic.) 3 Sept. 45/5: Thirteen clean shirts [...] three months hard labour.
[UK]Manchester Eve. News 4 Aug. 5/4: ‘Thirteen clean shirts,’ was the answer given, sotto voce.
[UK]Hull Dly Mail 7 Aug. 3/1: ‘It will mean thirteen clean shirts for me,’ said a man remanded at Marlborough-street Police Court [...] The police explained that this was the ‘technical’ term for three months or thirteen weeks imprisonment.
Blefast News Letter 1 July 14/2: ‘Got thirteen clean shirts for a bit of finger smithing’.
London Mercury 33 494: Slim Ben wiff thirteen clean shirts — three months in quod — and all ’acause ’e — Sweeny — called hot beef, set the cops on ’im for snatchin’.
[UK]Coventry Eve. Teleg. 23 July 4/2: ‘Thirteen clean shirts’ [...] three months’ hard labour.