Green’s Dictionary of Slang

1. a jay.

c.1786

Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Jacob, a Jay.

2. a fool.

a.1796		‘The Patriarch’ in Farmer Merry Songs and Ballads (1897) I 257: As honest Jacob on a night [...] Was duly laid on wedlock’s bed, / And noddin’ at his duty, / ‘How lang,’ she says, ‘ye fumbling wretch, Will ye be fucking at it?’.
1811		Lex. Balatronicum.
1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.