A character in Monstrous Regiment, Jack Jackrum is an immensely fat, hard-bitten tobacco chewing Borogravian sergeant with decades of military experience. His character appears to be at least partly based on Stanislaus Katczinsky, 'Kat' from Erich Maria Remarque's All Quiet on the Western Front. Like Kat, Jackrum acts as a father figure to the new recruits, taking them under his wing, teaching them all he knows, and going out of his way to procure food and comfortable accommodations for them. Both figures employ methods that are hardly official practice, but are effective and help keep their soldiers alive. Jackrum pretends to be a dimwitted soldier in the presence of senior officers but is really both intelligent and manipulative. The sergeant's exact age is unknown, as is his service record - lost owing to a series of baffling administrative errors - leaving only a legend. The legend creates a personna that is known, either personally or by reputation, to practically every soldier in the Borogravian Army. Jackrum boasts that he is probably quite as well known by the soldiers of the enemy armies too, up until the time that he kills them. Jackrum has, over the years, been the sergeant in command of (or under) a number of young soldiers who subsequently rose up to the Army's high command, and thus he wields considerable influence. It is stated on several occasions that Jackrum should have actually retired long ago, with his official resignation papers constantly following him around by mail, but he always finds some means to avoid actually retiring; the hold he has over senior officers revealed later in the novel. At one point in the book, he resigns his commission so that he can brutally assault an enemy soldier as a civilian without violating military protocol and is subsequently re-enlisted afterwards. Jackrum trains Polly Perks and the others in the #10 Foot (the Ins and Outs or the Cheesemongers) and gradually earns the respect of all the recruits.

Jackrum leads his 'men' in retaking the keep of the castle from the enemy, which eventually results in a truce but not before the gender of the regiment is revealed and they are imprisoned as 'Abominations under Nuggan' (the country's loony god). To secure their release, Jackrum confronts the heads of the Borogravian army and reveals (after asking the other two-thirds to depart the room) that almost a third of the commanders are women, (the young soldiers mentioned above who rose through the ranks and who he uncovered during his time in the army). He has kept their details in a diary for blackmail purposes should that be required, which he passes on to Polly at the end of the novel - the explanation of why he has never been forced to retire.

Jackrum too turns out to be a woman. Her real story (rather than the legend) was that about three generations ago, in rural Borogravia (which is all the Borogravia there is), a big, stroppy farm girl who married a soldier named William. When he went off to war, she soon tired of looking after the farm while her lazy brothers did nothing. She cut off her hair, acquired some trousers, and enlisted in the same regiment as William. The arrangement worked fairly smoothly for a time until William was killed next to her in battle, leaving the young Jackrum pregnant, something that she covered up by taking her considerable accumulated leave and then leaving the baby with her grandmother. The girl had already risen in the ranks, showing considerable military skill and leadership, and no one was offering widows' pensions, so she remained, and remained, until she was the most experienced and widely known NCO in the army. In the course of this long career, she discovered that she was not the only woman in her situation and helps them get ahead in their careers, often saving their lives and recording all the details in her little diary. Like the other women recruits, she has chosen a manly sounding name as her alias; in this case 'Jack' is slang for a labourer (steeplejack, lumberjack), or sailor (jack tar) and 'rum' has manly connotations being the drink of both army and navy (rum rations). When the novel ends, Jackrum takes Polly's advice and reunites with her long-lost son , introducing herself as his father rather than his mother, on the grounds that a fat old tobacco spitting woman showing up claiming to be his mother would just be an inconvenience, but a distinguished sergeant-major claiming to be his father would be something of which to be proud. She retired with the assurance that Polly Perks would carry on her leadership of the rude equal-opportunity movement.

Jackrum only reveals her true identity when Polly confronts her about the neck-locket containing an iconograph (photo) of William and Jackrum when young) which she shows to Polly on condition of secrecy.

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