Green’s Dictionary of Slang

mort n.

also maut
[ety. unknown; ? SE mort, a salmon in its third year, i.e. the popular equation of women with fish; Ribton-Turner, A History of Vagrants (1887), suggests Welsh modryb, a matron, morwyn, a virgin]

[mid-16C–1930s] a woman, esp. a prostitute.

In compounds

mort dell (n.)

[late 16C–19C] an unmarried woman or virgin girl who accompanies a mendicant villain.

mort wap-apace (n.) [wap v. and SE apace according to B.E.; but Dekker, O Per Se O (1612), a primary source for B.E., suggests that the phr. was anecdotal, noting that ‘there was an abram, who called his mort Madam Wap-apace’; all subseq. uses are glossarial]

[17C–early 19C] (UK Und.) an experienced prostitute or sexually active woman.