Green’s Dictionary of Slang

boot n.5

[SE boot (US trunk), i.e. of an automobile]
(N.Z. prison)

the rectum.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 27/1: boot n. the rectum.

In derivatives

bootload (n.)

a large volume of drugs or other contraband as carried in the rectum or vagina.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 27/2: bootload n. 1 a good-sized charge, a substantial package of contraband concealed in the vagina or rectum [...] 2 a large amount of drugs.
bootman (n.)

one who moves contraband around the prison by smuggling items in his rectum.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 27/1: bootman an inmate who passes contraband between prison blocks, wings or units by smuggling the goods in his rectum.