Green’s Dictionary of Slang

jankers n.

(UK milit.) punishment for defaulters.

[UK]Regiment 27 Jan. 288/1: [W]hen a soldier has received punishment and is sentenced to so many days ‘C.B.’ (confined to barracks), he is said to be doing ‘jankers’ .

In compounds

In phrases

on jankers

in prison or undergoing some form of punitive discipline.

[UK](con. 1900s) F. Richards Old Soldier Sahib (1965) 54: I was now a defaulter, or ‘on jankers’ as the troops called it.
[UK]J. Maclaren-Ross Of Love and Hunger 211: And when you’re sweating on jankers or cookhouse, just remember yours truly, Larry Heliotrope!
[SA]J. Yates-Benyon Weak and the Wicked 51: Words such as [...] ‘jankers’, ‘adrift’ and ‘hatter’ were well-worn synonyms for [...] ‘punishments’, ‘desertion’, and ‘homosexual’.
[UK](con. WWII) B. Aldiss Soldier Erect 90: Someone was reminiscing gaily about the first time he was on jankers. Mention of jankers reminded me of picket duty.
[UK]G.M. Fraser McAuslan in the Rough 13: I don’t care if half the detachment’s on jankers.
[UK](con. WW2) T. Jones Heart of Oak [ebook] Get your plates moving or I’ll have the slew of you on bloody jankers.
[UK]J. Meades Empty Wigs (t/s) 426: ‘What’s jankers.’ ‘Squaddy lingo idn’t it. When you been a bad lad you get jankers. Punishment’.