Green’s Dictionary of Slang

kelp n.¹

a hat.

1753		J. Poulter Discoveries (1774) 30: We jostle him up, and one knocks his Kelp off, and while he lifts his Hand up, his Loag is napp’d. [Ibid.] 43: Calp, Noll, and Smish; Hat, Wig, and Shirt.
1786		Whole Art of Thieving.
1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.