Green’s Dictionary of Slang

1. an unpleasant person, thus a thug [a cur].

1619		Fletcher Monsieur Thomas (1639) II ii: I’le not be bob’d i’ th’ nose with every bobtail .
1672		J. Phillips Maronides (1678) VI 98: Which grim Judge hearing sends his bob-tails / To comb his tawnie skin with Hob-nails.

2. an impotent man, a eunuch.

1660		Man in the Moon 4 26 Nov. 28: Kittermaster (alias quite Whore-master) [whose] shortness is caused by the long continuance of his wenching, no wonder then if he be call’d Bob, to which if you add Tale it will better discover his Trade into the Antipodes.
1725		New Canting Dict. n.p.: bob-tail [...] also an Eunuch or impotent Fellow.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.

3. a prostitute; thus bobtailed adj. [cf. SE bob, go up and down + tail n. (2); a horse is also good for a ride n. (1a)].

1654		T. Killigrew Thomaso Pt II III ii: I hate such hawking at the Bush with those Bobtails; Give me a long-wing’d Hawke, a high flying whore, ’tis sport to see her stoop.
1673		J. Wade Vinegar and Mustard B: Do you remember how thou layst with a Fisher-man for a quardern of Mackarel, and when you came back agen how you paid the Water-man with a pox that carried you, thou bobtail’d Whore thou.
c.1698		B.E. Dict. Canting Crew n.p.: Bob-tail, a light Woman.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1759–67		Sterne Tristram Shandy (1949) 334: There was a good farcical house, large enough to hold – aye – and sublimate them, shag-rag and bob-tail, male and female, all together.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Bob tail, (cant) a lewd woman, or one that plays with her tail.
a.1800		‘The Bob-Tail’d Lass’ in Farmer Merry Songs and Ballads (1897) I 276: I was never so disgraced in my life / As I was by this bob-tail’d whoore.
1811		Lex. Balatronicum.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.
2024		J. Meades Empty Wigs (t/s) 209: The pretty popsy bobtail must have agreed without much hesitation to their offer.