Green’s Dictionary of Slang

1. (US) a prostitute.

1848		Satirist & Blade (Boston, MA) 19 Feb. n.p.: When you meet a ‘lady’ with painted face, a flashy bonnet, and impudent stare, especially if she swings her rear when walking [...] you may safely set her down as a member of the frail issterhood, i.e. a shad.
1851		Life in Boston & N.Y. (Boston, MA) 28 June n.p.: A notorious shad met one of her companions [...] who asked her how business had been with her. ‘First rate [...] I cleared five dollars last saturday night’.
1855		Life in Boston & N.Y. (Boston, MA) 14 Apr. n.p.: Why did Cad Osgood leave Ada Dean’s? Was it because she got disgusted with the ‘shad’ suppers?

2. (US und.) a railroad brakeman.

1909

‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] The brakeman [...] is termed a bakery, a Christmas, a shad or a shack.