Green’s Dictionary of Slang

1. a prostitute.

c.1591		Shakespeare Comedy of Errors III ii: O, train me not, sweet mermaid, with thy note [...] Spread o’er the silver waves thy golden hairs, And as a bed I’ll take them and there lie.
1611		Middleton & Dekker Roaring Girle I i: Nature repents she made her: ’tis a mermaid Has toled my son to shipwreck.
1615		R. Brathwait Strappado 48: A wanton Meremayd, that does sing, To bring youths crazie backe to ruining.
c.1619		Rowley All’s Lost by Lust III iii: There are fowle, and there are fish, there are wag-tayles, and there are Mermayds.
1881		Bulletin (Sydney) 23 Apr. 9/1: [title] ‘The Mermaid of the Block’.
1908		G.R. Bacchus Pleasure Bound ‘Afloat’ (1969) 132: ‘A mermaid [...] is a dear, delightful dot of dimity, who doesn’t exactly traverse this boundless waste of wave because she loves it, but because there are gents like you, sir, who have money to spend and want a little occasional diversion’.
1913		Sport (Adelaide) 15 Feb. 11/3: They Say [...] That Jack N [...] is going up Fishtown [...] fishing for mermaids.

2. (N.Z.) a police officer at a weigh station [see cit.] .

2003

McGill Reed Dict. of N.Z. Sl. 134: mermaids The police at weigh stations, because they are ‘cunts with scales’. ANZ.